Strontium magnesium phosphate, Sr_2+xMg_3-xP₄O₁₅ (x ~ 0.36), from laboratory X-ray powder data

The previously unknown crystal structure of strontium mag­nesium phosphate, Sr_2+xMg_3-xP₄O₁₅ (x ~ 0.36), determined and refined from laboratory powder X-ray diffraction data, represents a new structure type. The title compound was synthesized by high-temperature solid-state reaction and it crystallizes in the ortho­rhom­bic space group Cmcm. It was earlier thought to be stoichiometric Sr₂Mg₃P₄O₁₅, but our structural study indicates the nonstoichiometric composition. The asymmetric unit contains one Sr (site symmetry ..m on special position 8g), one M (= Mg 64%/Sr 36%; site symmetry 2/m.. on special position 4b), one Mg (site symmetry 2.. on special position 8e), two P (site symmetry m.. on special position 8f and site symmetry ..m on special position 8g), and six O sites [two on general positions 16h, two on 8g, one on 8f and one on special position 4c (site symmetry m2m)]. The nonstoichiometry is due to the mixing of magnesium and strontium ions on the M site. The structure consists of three-dimensional networks of MgO₄ and PO₄ tetra­hedra, and MO₆ octa­hedra with the other strontium ions occupying the larger cavities surrounded by ten O atoms. All the polyhedra are connected by corner-sharing except the edge-sharing MO₆ octa­hedra forming one-dimensional arrangements along [001].

S1. Comment

Sr₂Mg₃P₄O₁₅:Eu²⁺ was recently reported to show potential for application as a blue phosphor for a white LED under excitation of near-UV light (Ngee et al., 2009; Guo et al., 2010). The existence of a single phase in the system SrO–MgO–P₂O₅ was first confirmed over 40 years ago (Hoffman, 1968), but only un-indexed X-ray diffraction data were reported and its crystal structure has remained unknown. In the present study, during the course of crystal structure determination we have discovered that the compound previously known as Sr₂Mg₃P₄O₁₅ is in fact nonstoichiometric Sr_2+xMg_3-xP₄O₁₅ (x~0.36). Here we present its crystal structure, as determined and refined from laboratory powder X-ray diffraction data (Fig. 1).

Sr_2+xMg_3-xP₄O₁₅ crystallizes in a new structure type in terms of atomic ratios (6:1:2:15 for tetrahedral:octahedral:ten-coordinated metal:oxygen) and its polyhedral network is, to our knowledge, unique. The structure consists of (Mg2)O₄, (P1)O₄, and (P2)O₄ tetrahedra, and MO₆ octahedra, where M (≡ Mg1/Sr2) represents disordered magnesium (Mg1, 64%) and strontium (Sr2, 36%) ions (Fig. 2). The other strontium (Sr1) ions occupy the larger cavities surrounded by O atoms with tenfold coordination (Fig. 3). Each (Mg2)O₄ tetrahedron is engaged in corner-sharing with two (P1)O₄ and two (P2)O₄ tetrahedra. (P1)O₄ corner-shares with another (P1)O₄ (to form a P₂O₇ group), two (Mg2)O₄ and one MO₆ polyhedra. (P2)O₄ is connected to only two polyhedra: (Mg2)O₄ and MO₆. Each O6 atom in the other two corners of the P2 tetrahedron is bonded to two Sr1 ions, one in common to both O atoms. The MO₆ octahedra form one-dimensional arrangements along [001], sharing edges. Each MO₆ is also engaged in corner-sharing with two opposite-sided (P1)O₄ tetrahedra along [010] and four (P2)O₄ tetrahedra parallel to the (010) plane. The average M—O distance of 2.265 Å is very close to the expected value from the sum of M and O ionic radii (2.27Å) weighted by 64% Mg and 36% Sr (Shannon, 1976).

The empirical expression for bond valence, which has been widely adopted to estimate valences in inorganic solids (Brown, 2002), was used to check the Sr_2+xMg_3-xP₄O₁₅ (x~0.36) crystal structure. The bond-valence sums (Brown & Altermatt, 1985; Brese & O'Keeffe, 1991) calculated with the program VaList (Wills, 2010) for Sr1 (2.04 v.u.), (Mg1/Sr2) (2.30), Mg2 (2.11), P1 (5.06), P2 (4.86), O1 (2.09), O2 (2.12), O3 (2.00), O4 (2.12), O5 (2.20) and O6 (1.85) match the expected charges of the ions reasonably well. The higher valence sum for (Mg1/Sr2) resulted from the shorter Sr2—O bond distances (2.265 Å). All the other interatomic distances (Table 1) are within the expected ranges.

The nonstoichiometry model with more strontium and fewer magnesium ions also conformed to the synthesis results that magnesium phosphates were the impurity phases. Several trials to prepare a single phase Sr_2+xMg_3-xP₄O₁₅ (x~0.36) with the nonstoichiometric nominal composition under the same synthesis conditions were unsuccessful: the magnesium phosphate impurities disappeared but instead SrMgP₂O₇ (Tahiri et al., 2002) appeared as an impurity phase in the X-ray diffraction pattern. The relative atomic stoichiometries of Sr_2.36Mg_2.64P₄O₁₅ and SrMgP₂O₇ are quite similar and they seemed to compete with each other kinetically and/or thermodynamically in our synthetic condition. More careful work should be undertaken to determine a reproducible synthetic condition for the single phase, which is beyond the purpose of the present study.

The crystal structure of SrMgP₂O₇ is rather simple compared to Sr_2.36Mg_2.64P₄O₁₅. It consists of PO₄ tetrahedra, MgO₅ square-pyramids and eight-coordinated strontium ions occupying the larger cavities surrounded by O atoms. Each PO₄ corner-shares with another PO₄ [to form a P₂O₇ group, similar to (P1)O₄ in Sr_2.36Mg_2.64P₄O₁₅] and three MgO₅ polyhedra. All five corners of MgO₅ are connected to PO₄ tetrahedra. There is no MgO₄ tetrahedron or octahedron in this structure, unlike Sr_2.36Mg_2.64P₄O₁₅.

Related literature

For related literature, see: Betteridge et al. (2003); Boultif & Louër (1991); Brese & O'Keeffe (1991); Brown (2002); Brown & Altermatt (1985); Calvo (1967); Guo et al. (2010); Hoffman (1968); Larson & Von Dreele (2000); Lee & Hong (2008); Ngee et al. (2009); Nord & Kierkegaard (1968); Rohlíček & Hušák (2007); Shannon (1976); Sheldrick (2008); Shirley (2002); Tahiri et al. (2002); Wills (2010).

S2. Experimental

Sr_2+xMg_3-xP₄O₁₅ (x~0.36) was synthesized by a solid-state reaction from a mixture of high-purity SrCO₃ (99.994%, Alfa Aesar), MgO (99.0%, Yakuri Pure Chemicals) and (NH₄)₂HPO₄ (99.0%, Junsei Chemical) with a nominal composition of (Sr: Mg: P = 2: 3: 4). The mixture was thoroughly ground in an agate mortar, dried, pressed into a pellet, heated in air at 1303 K for 4 h, and again at 1323 K for 4 h with intermediate grinding and pressing. The yield was about 90% by weight. The nonstoichiometric composition of the major phase, Sr_2+xMg_3-xP₄O₁₅ (x~0.36), was determined later by the structural refinement, and the minor impurity phases were determined to be Mg₂P₂O₇ (Calvo, 1967) and Mg₃P₂O₈ (Nord & Kierkegaard, 1968).

S3. Refinement

The powder X-ray diffraction (XRD) data were collected at room temperature on a Bragg–Brentano diffractometer (Bruker AXS Advance D8) with a Cu X-ray tube, a focusing primary Ge (111) monochromator (λ = 1.5406 Å) and a position-sensitive Väntec detector with a 6° slit. Data acquisition covered the angular range 8° ≤ 2θ ≤ 140° at a step width of 0.016682° and a total measurement time of 40 h. The structure determination from the powder XRD data was performed using a combination of the powder profile refinement program GSAS (Larson & Von Dreele, 2000) and the single-crystal structure refinement program CRYSTALS (Betteridge et al., 2003). For a three-dimensional view of the Fourier density maps, MCE was used (Rohlíček & Hušák, 2007). The XRD pattern was indexed using the program DICVOL91 (Boultif & Louër, 1991) run in CRYSFIRE (Shirley, 2002) via the positions of 20 diffraction peaks after excluding the impurity peaks, resulting in an orthorhombic unit cell. The systematic absences suggested three possible space groups: Cmc2₁, C2cm and Cmcm. All of them would have resulted in basically the same structure, thus the space group Cmcm with the highest symmetry was chosen for the final refinement. LeBail fitting was carried out for the previously unknown phase of Sr_2+xMg_3-xP₄O₁₅, while Rietveld fitting was carried out for the two known impurity phases.

The structure determination was performed in the same way as in our previous work (Lee & Hong, 2008) where the details were described. At the beginning, a structural model with only a dummy atom at an arbitrary position in the unit cell was used. Structure factors were extracted from the powder data, then direct methods were used for the initial solution of the structure using SHELXS (Sheldrick, 2008) run in CRYSTALS, which yielded several metal positions. However, not all the atoms could be identified at once. The partial model at this stage replaced the initial dummy-atom model, and was used for a LeBail fit in GSAS. Then, improved structure factors were extracted, which were used for the improved data in the refinement in CRYSTALS. These processes were iterated until a complete and satisfactory structural model was obtained. Finally, Rietveld refinement was employed to complete the structure determination. Up to this step, a stoichiometric composition of Sr₂Mg₃P₄O₁₅ was assumed for the unknown phase, and indeed the crystallographic sites ratio seemed to conform to the stoichiometry. However, the thermal parameter of the Mg1 site went to a very small value, and Mg1—O distances (circa 2.3 Å) were longer than the expected (2.10 Å), but much shorter than the 2.56 Å expected for d(Sr—O) from ionic radii (Shannon, 1976). Therefore, it was assumed that some magnesium is substituted by strontium in the Mg1 site, and Sr2 and Mg1 occupancies were refined with a constraint that the thermal parameters for M (Mg1/Sr2) and Mg2 sites were the same, resulting in a dramatic improvement in refinement (wR_p factors from 16.8 to 6.3%) with reasonable thermal parameters. Lowering the symmetry to Cmc2₁ or C2cm did not separate the problematic Mg1 site, and thus should not make any difference to this outcome. For the impurity phases, only the cell parameters and scale factors were refined while the other variables were fixed in the final refinement. The overall fit (wR_p = 6.3%) is shown in Fig. 1.

Computing details

For all compounds, data collection: COMMANDER (Bruker, 2003); cell refinement: GSAS (Larson & Von Dreele, 2000); data reduction: EVA (Bruker, 2003); program(s) used to solve structure: SHELXS (Sheldrick, 2008) and CRYSTALS (Betteridge et al., 2003); program(s) used to refine structure: GSAS (Larson & Von Dreele, 2000); molecular graphics: ATOMS (Dowty, 2000); software used to prepare material for publication: GSAS (Larson & Von Dreele, 2000).

Figures

	Fig. 1. X-ray Rietveld refinement profiles for Sr2+xMg3-xP4O15 (x~0.36) including the minor impurity phases, recorded at room temperature. The crossed line marks experimental points and the solid line is the calculated profile. The bottom trace shows the difference curve, and the ticks denote expected peak positions for Sr2+xMg3-xP4O15 (x~0.36), Mg2P2O7 and Mg3P2O8, respectively, in order from the bottom.
	Fig. 2. Views of the structure along (001) (a) and (010) (b) with the unit cell outlined.
	Fig. 3. The local environment of strontium (Sr1). [Symmetry codes: (i) x, 1 - y, -z; (ii) x, 1 - y, 1/2 + z; (iii) x, y, 1/2 - z; (iv) 1/2 - x, 1/2 + y, z; (v) 1/2 - x, y – 1/2, z; (vi) 1/2 - x, y – 1/2, 1/2 – z]

(phase_1) distrontium dimagnesium pentadecaoxidotetraphosphate

Crystal data

Sr2.36Mg2.64P4O15	Z = 4
Mr = 634.65	F(000) = 1204.4
Orthorhombic, Cmcm	Dx = 3.490 Mg m^−3
Hall symbol: -C 2c 2	Cu Kα1 radiation, λ = 1.5406 Å
a = 14.27366 (9) Å	T = 296 K
b = 11.75678 (7) Å	white
c = 7.19753 (4) Å	flat sheet, 20 × 20 mm
V = 1207.83 (1) Å^3	Specimen preparation: Prepared at 296 K

Data collection

Bruker D8 Advance diffractometer	Data collection mode: reflection
Radiation source: sealed X-ray tube, Bruker Cu Ceramic X-ray tube	Scan method: step
Ge 111 monochromator	2θmin = 8.0°, 2θmax = 140.422°, 2θstep = 0.017°
Specimen mounting: packed powder pellet

Refinement

Least-squares matrix: full	7939 data points
Rp = 0.048	Profile function: CW Profile function number 4 with 18 terms Pseudovoigt profile coefficients as parameterized in Thompson et al. [Thompson, P., Cox, D. E. & Hastings, J. B. (1987). J. Appl. Cryst. 20, 79–83]. Asymmetry correction of Finger et al. [Finger, L. W., Cox, D. E. & Jephcoat, A. P. (1994). J. Appl. Cryst. 27, 892–900]. Microstrain broadening by Stephens [Stephens, P. W. (1999). J. Appl. Cryst. 32, 281–289]. #1(GU) = 101.674 #2(GV) = -32.707 #3(GW) = 2.792 #4(GP) = 11.704 #5(LX) = 5.714 #6(ptec) = 0.00 #7(trns) = 0.00 #8(shft) = 3.1799 #9(sfec) = 0.00 #10(S/L) = 0.0087 #11(H/L) = 0.0005 #12(eta) = 0.7500 #13(S400 ) = 0.0E+00 #14(S040 ) = 0.0E+00 #15(S004 ) = 0.0E+00 #16(S220 ) = 0.0E+00 #17(S202 ) = 0.0E+00 #18(S022 ) = 0.0E+00 Peak tails are ignored where the intensity is below 0.0020 times the peak Aniso. broadening axis 0.0 0.0 1.0
Rwp = 0.063	81 parameters
Rexp = 0.025	0 restraints
RBragg = 0.047	Weighting scheme based on measured s.u.'s
R(F) = 0.028	(Δ/σ)max = 0.02
R(F^2) = 0.04372	Background function: GSAS Background function number 1 with 36 terms. Shifted Chebyshev function of 1st kind 1: 667.815 2: -121.026 3: 250.928 4: -108.029 5: 130.095 6: -120.393 7: 81.5238 8: -66.4354 9: 31.4946 10: -19.2674 11: 16.0150 12: -21.1906 13: 24.6787 14: -7.73173 15: 5.82669 16: 7.16351 17: 9.66948 18: 8.02466 19: 5.65618 20: 4.74674 21: -11.8793 22: -0.873263 23: -2.40368 24: -9.55052 25: -6.36605 26: 6.28370 27: -8.07052 28: 6.33955 29: -12.6008 30: 3.68005 31: 5.95000 32: 7.62516 33: 1.58465 34: 5.63269 35: -2.90398 36: 14.7130
χ^2 = 6.250

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Sr1	0.16729 (5)	0.29123 (6)	0.25	0.0157 (2)*
Sr2	0.0	0.5	0.0	0.0185 (4)*	0.3571 (23)
Mg1	0.0	0.5	0.0	0.0185 (4)*	0.6429 (23)
Mg2	0.16691 (18)	0.0	0.0	0.0185 (4)*
P1	0.0	0.80342 (16)	0.0344 (3)	0.0148 (4)*
P2	0.19114 (12)	0.57060 (18)	0.25	0.0148 (4)*
O1	0.0	0.6879 (4)	−0.0498 (5)	0.0250 (5)*
O2	0.0979 (3)	0.5046 (4)	0.25	0.0250 (5)*
O3	0.08811 (19)	0.1331 (2)	0.0171 (4)	0.0250 (5)*
O4	0.2279 (3)	−0.0215 (4)	0.25	0.0250 (5)*
O5	0.0	0.7839 (6)	0.25	0.0250 (5)*
O6	0.2059 (2)	0.6400 (3)	0.0769 (4)	0.0250 (5)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
?	?	?	?	?	?	?

Geometric parameters (Å, º)

Sr1—O1^i	2.7998 (19)	P1—O3^ii	1.509 (3)
Sr1—O1^ii	2.7998 (19)	P1—O3^v	1.509 (3)
Sr1—O2	2.697 (4)	P1—O5	1.5684 (19)
Sr1—O3	2.747 (3)	P2—Mg2^iv	2.834 (2)
Sr1—O3^iii	2.747 (3)	P2—Mg2^xv	2.834 (2)
Sr1—O4^iv	2.662 (4)	P2—O2	1.541 (4)
Sr1—O6^i	2.548 (3)	P2—O4^iv	1.584 (5)
Sr1—O6^v	2.548 (3)	P2—O6	1.504 (3)
Sr1—O6^vi	2.827 (3)	P2—O6^iii	1.504 (3)
Sr1—O6^vii	2.827 (3)	O1—Sr1^ix	2.7998 (19)
Sr2—O1	2.238 (4)	O1—Sr1^x	2.7998 (19)
Sr2—O1^ii	2.238 (4)	O1—Sr2	2.238 (4)
Sr2—O2	2.279 (3)	O1—Mg1	2.238 (4)
Sr2—O2^viii	2.279 (3)	O1—P1	1.487 (4)
Sr2—O2^ix	2.279 (3)	O2—Sr1	2.697 (4)
Sr2—O2^x	2.279 (3)	O2—Sr2	2.279 (3)
Sr2—O6	3.414 (3)	O2—Sr2^i	2.279 (3)
Sr2—O6^viii	3.414 (3)	O2—Mg1	2.279 (3)
Sr2—O6^ii	3.414 (3)	O2—Mg1^i	2.279 (3)
Sr2—O6^v	3.414 (3)	O2—P2	1.541 (4)
Mg1—O1	2.238 (4)	O3—Sr1	2.747 (3)
Mg1—O1^ii	2.238 (4)	O3—Mg2	1.931 (3)
Mg1—O2	2.279 (3)	O3—P1^ii	1.509 (3)
Mg1—O2^viii	2.279 (3)	O4—Sr1^vi	2.662 (4)
Mg1—O2^ix	2.279 (3)	O4—Mg2	2.015 (2)
Mg1—O2^x	2.279 (3)	O4—Mg2^xvi	2.015 (2)
Mg2—P2^vi	2.834 (2)	O4—P2^vi	1.584 (5)
Mg2—P2^xi	2.834 (2)	O5—P1	1.5684 (19)
Mg2—O3	1.931 (3)	O5—P1^xvii	1.5684 (19)
Mg2—O3^xii	1.931 (3)	O6—Sr1^ix	2.548 (3)
Mg2—O4	2.015 (2)	O6—Sr1^iv	2.827 (3)
Mg2—O4^xiii	2.015 (2)	O6—Sr2	3.414 (3)
Mg2—O6^vi	2.512 (3)	O6—Mg2^iv	2.512 (3)
Mg2—O6^xiv	2.512 (3)	O6—P2	1.504 (3)
P1—O1	1.487 (4)
O1^i—Sr1—O1^ii	61.95 (13)	O1—Mg1—O2^x	84.08 (13)
O1^i—Sr1—O2	66.74 (12)	O1^ii—Mg1—O2	84.08 (13)
O1^i—Sr1—O3	91.29 (10)	O1^ii—Mg1—O2^viii	84.08 (13)
O1^i—Sr1—O3^xviii	52.72 (9)	O1^ii—Mg1—O2^ix	95.92 (13)
O1^i—Sr1—O4^iv	114.01 (11)	O1^ii—Mg1—O2^x	95.92 (13)
O1^i—Sr1—O6^i	71.43 (9)	O2—Mg1—O2^viii	75.61 (16)
O1^i—Sr1—O6^v	129.19 (10)	O2—Mg1—O2^ix	104.39 (16)
O1^i—Sr1—O6^vi	145.90 (11)	O2—Mg1—O2^x	180.0
O1^i—Sr1—O6^xix	111.99 (10)	O2^viii—Mg1—O2^ix	179.9802
O1^ii—Sr1—O2	66.74 (12)	O2^viii—Mg1—O2^x	104.39 (16)
O1^ii—Sr1—O3	52.72 (9)	O2^ix—Mg1—O2^x	75.61 (16)
O1^ii—Sr1—O3^xviii	91.29 (10)	O3—Mg2—O3^xii	108.7 (2)
O1^ii—Sr1—O4^iv	114.01 (11)	O3—Mg2—O4	107.24 (14)
O1^ii—Sr1—O6^i	129.19 (10)	O3—Mg2—O4^xiii	101.93 (15)
O1^ii—Sr1—O6^v	71.43 (9)	O3^xii—Mg2—O4	101.93 (15)
O1^ii—Sr1—O6^vi	111.99 (10)	O3^xii—Mg2—O4^xiii	107.24 (14)
O1^ii—Sr1—O6^xix	145.90 (11)	O4—Mg2—O4^xiii	128.8 (3)
O2—Sr1—O3	118.58 (9)	O1—P1—O3^ii	110.58 (17)
O2—Sr1—O3^xviii	118.58 (9)	O1—P1—O3^v	110.58 (17)
O2—Sr1—O4^iv	55.77 (13)	O1—P1—O5	105.6 (3)
O2—Sr1—O6^i	77.54 (8)	O3^ii—P1—O3^v	112.9 (3)
O2—Sr1—O6^v	77.54 (8)	O3^ii—P1—O5	108.4 (2)
O2—Sr1—O6^vi	145.12 (9)	O3^v—P1—O5	108.4 (2)
O2—Sr1—O6^xix	145.12 (9)	O2—P2—O4^iv	106.6 (3)
O3—Sr1—O3^xviii	75.21 (12)	O2—P2—O6	113.22 (17)
O3—Sr1—O4^iv	142.30 (6)	O2—P2—O6^xviii	113.22 (17)
O3—Sr1—O6^i	150.15 (10)	O4^iv—P2—O6	105.57 (16)
O3—Sr1—O6^v	74.96 (8)	O4^iv—P2—O6^xviii	105.57 (16)
O3—Sr1—O6^vi	64.44 (8)	O6—P2—O6^xviii	111.9 (3)
O3—Sr1—O6^xix	96.12 (8)	Sr1^ix—O1—Sr1^x	117.05 (13)
O3^xviii—Sr1—O4^iv	142.30 (6)	Sr1^ix—O1—Sr2	99.73 (11)
O3^xviii—Sr1—O6^i	74.96 (8)	Sr1^ix—O1—Mg1	99.73 (11)
O3^xviii—Sr1—O6^v	150.15 (10)	Sr1^ix—O1—P1	97.45 (13)
O3^xviii—Sr1—O6^vi	96.12 (8)	Sr1^x—O1—Sr2	99.73 (11)
O3^xviii—Sr1—O6^xix	64.44 (8)	Sr1^x—O1—Mg1	99.73 (11)
O4^iv—Sr1—O6^i	67.41 (7)	Sr1^x—O1—P1	97.45 (13)
O4^iv—Sr1—O6^v	67.41 (7)	Sr2—O1—P1	146.7 (3)
O4^iv—Sr1—O6^vi	99.23 (11)	Mg1—O1—P1	146.7 (3)
O4^iv—Sr1—O6^xix	99.23 (11)	Sr1—O2—Sr2	101.73 (13)
O6^i—Sr1—O6^v	134.82 (14)	Sr1—O2—Sr2^i	101.73 (13)
O6^i—Sr1—O6^vi	117.91 (7)	Sr1—O2—Mg1	101.73 (13)
O6^i—Sr1—O6^xix	69.77 (10)	Sr1—O2—Mg1^i	101.73 (13)
O6^v—Sr1—O6^vi	69.77 (10)	Sr1—O2—P2	98.7 (2)
O6^v—Sr1—O6^xix	117.91 (7)	Sr2—O2—Sr2^i	104.32 (16)
O6^vi—Sr1—O6^xix	52.31 (12)	Sr2—O2—Mg1^i	104.32 (16)
O1—Sr2—O1^ii	179.9657	Sr2—O2—P2	122.79 (12)
O1—Sr2—O2	95.92 (13)	Sr2^i—O2—Mg1	104.32 (16)
O1—Sr2—O2^viii	95.92 (13)	Sr2^i—O2—P2	122.79 (12)
O1—Sr2—O2^ix	84.08 (13)	Mg1—O2—Mg1^i	104.32 (16)
O1—Sr2—O2^x	84.08 (13)	Mg1—O2—P2	122.79 (12)
O1^ii—Sr2—O2	84.08 (13)	Mg1^i—O2—P2	122.79 (12)
O1^ii—Sr2—O2^viii	84.08 (13)	Sr1—O3—Mg2	110.35 (13)
O1^ii—Sr2—O2^ix	95.92 (13)	Sr1—O3—P1^ii	99.10 (16)
O1^ii—Sr2—O2^x	95.92 (13)	Mg2—O3—P1^ii	150.5 (2)
O2—Sr2—O2^viii	75.61 (16)	Sr1^vi—O4—Mg2	110.29 (12)
O2—Sr2—O2^ix	104.39 (16)	Sr1^vi—O4—Mg2^xvi	110.29 (12)
O2—Sr2—O2^x	180.0	Sr1^vi—O4—P2^vi	98.9 (2)
O2^viii—Sr2—O2^ix	179.9802	Mg2—O4—Mg2^xvi	126.5 (3)
O2^viii—Sr2—O2^x	104.39 (16)	Mg2—O4—P2^vi	103.25 (15)
O2^ix—Sr2—O2^x	75.61 (16)	Mg2^xvi—O4—P2^vi	103.25 (15)
O1—Mg1—O1^ii	179.9657	P1—O5—P1^xx	163.2 (5)
O1—Mg1—O2	95.92 (13)	Sr1^ix—O6—Sr1^iv	110.23 (10)
O1—Mg1—O2^viii	95.92 (13)	Sr1^ix—O6—P2	155.0 (2)
O1—Mg1—O2^ix	84.08 (13)	Sr1^iv—O6—P2	93.78 (16)

Symmetry codes: (i) x, −y+1, z+1/2; (ii) −x, −y+1, −z; (iii) x, y, −z+1/2; (iv) −x+1/2, y+1/2, z; (v) x, −y+1, −z; (vi) −x+1/2, y−1/2, z; (vii) −x−1/2, y−3/2, −z+1/2; (viii) −x, y, z; (ix) x, −y+1, z−1/2; (x) −x, −y+1, z−1/2; (xi) −x+1/2, −y+1/2, z−1/2; (xii) x, −y, −z; (xiii) x, −y, z−1/2; (xiv) −x−1/2, −y−1/2, −z; (xv) −x+1/2, −y+1/2, z+1/2; (xvi) x, −y, z+1/2; (xvii) −x, y, −z+1/2; (xviii) x, y, −z+3/2; (xix) −x+1/2, y−1/2, −z+3/2; (xx) −x−1, y−1, −z+1/2.

(phase_2) dimagnesium heptaoxidodiphosphate

Crystal data

Mg2P2O7	Z = 4
Mr = 222.56	F(000) = 440.0
Monoclinic, P21/c	Dx = 3.105 Mg m^−3
Hall symbol: -P 2ybc	Cu Kα1 radiation, λ = 1.5406 Å
a = 6.9443 (4) Å	T = 296 K
b = 8.2861 (4) Å	white
c = 9.0438 (5) Å	flat sheet, 20 × 20 mm
β = 113.816 (3)°	Specimen preparation: Prepared at 296 K
V = 476.08 (5) Å^3

Data collection

Bruker D8 Advance diffractometer	Data collection mode: reflection
Radiation source: sealed X-ray tube, Bruker Cu Ceramic X-ray tube	Scan method: step
Ge 111 monochromator	2θmin = 8.0°, 2θmax = 140.422°, 2θstep = 0.017°
Specimen mounting: packed powder pellet

Refinement

Least-squares matrix: full	7939 data points
Rp = 0.048	Profile function: CW Profile function number 4 with 21 terms Pseudovoigt profile coefficients as parameterized in Thompson et al. [Thompson, P., Cox, D. E. & Hastings, J. B. (1987). J. Appl. Cryst. 20, 79–83]. Asymmetry correction of Finger et al. [Finger, L. W., Cox, D. E. & Jephcoat, A. P. (1994). J. Appl. Cryst. 27, 892–900]. Microstrain broadening by Stephens [Stephens, P. W. (1999). J. Appl. Cryst. 32, 281–289]. #1(GU) = 178.981 #2(GV) = -107.291 #3(GW) = 12.592 #4(GP) = 19.425 #5(LX) = 3.011 #6(ptec) = 0.00 #7(trns) = 0.00 #8(shft) = 3.1799 #9(sfec) = 0.00 #10(S/L) = 0.0005 #11(H/L) = 0.0005 #12(eta) = 0.7500 #13(S400 ) = 0.0E+00 #14(S040 ) = 0.0E+00 #15(S004 ) = 0.0E+00 #16(S220 ) = 0.0E+00 #17(S202 ) = 0.0E+00 #18(S022 ) = 0.0E+00 #19(S301 ) = 0.0E+00 #20(S103 ) = 0.0E+00 #21(S121 ) = 0.0E+00 Peak tails are ignored where the intensity is below 0.0020 times the peak Aniso. broadening axis 0.0 0.0 1.0
Rwp = 0.063	81 parameters
Rexp = 0.025	0 restraints
RBragg = 0.047	Weighting scheme based on measured s.u.'s
R(F) = 0.047	(Δ/σ)max = 0.02
R(F^2) = 0.04372	Background function: GSAS Background function number 1 with 36 terms. Shifted Chebyshev function of 1st kind 1: 667.815 2: -121.026 3: 250.928 4: -108.029 5: 130.095 6: -120.393 7: 81.5238 8: -66.4354 9: 31.4946 10: -19.2674 11: 16.0150 12: -21.1906 13: 24.6787 14: -7.73173 15: 5.82669 16: 7.16351 17: 9.66948 18: 8.02466 19: 5.65618 20: 4.74674 21: -11.8793 22: -0.873263 23: -2.40368 24: -9.55052 25: -6.36605 26: 6.28370 27: -8.07052 28: 6.33955 29: -12.6008 30: 3.68005 31: 5.95000 32: 7.62516 33: 1.58465 34: 5.63269 35: -2.90398 36: 14.7130
χ^2 = 6.250

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Mg1	0.24957	0.92491	0.12118	0.02084*
Mg2	0.69861	0.43311	0.82859	0.02084*
P1	0.94785	0.76438	0.76493	0.02084*
P2	0.52451	0.77297	0.46834	0.02084*
O1	0.72902	0.83262	0.59014	0.02084*
O2	0.37623	0.76109	0.55127	0.02084*
O3	1.12756	0.76909	0.69986	0.02084*
O4	1.01129	0.90711	0.89533	0.02084*
O5	0.92364	0.5963	0.82874	0.02084*
O6	0.47096	0.89728	0.32033	0.02084*
O7	0.59983	0.60601	0.4192	0.02084*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
?	?	?	?	?	?	?

Geometric parameters (Å, º)

Mg1—O2^i	1.9994 (1)	O1—P1	1.7854 (1)
Mg1—O3^ii	2.0725 (1)	O1—P2	1.4870 (1)
Mg1—O4^iii	2.0499 (1)	O2—Mg1^viii	1.9994 (1)
Mg1—O4^iv	2.2412 (1)	O2—Mg2^x	1.9845 (1)
Mg1—O6	1.8503 (1)	O2—P2	1.5021 (1)
Mg1—O7^v	1.9461 (1)	O3—Mg1^xi	2.0725 (1)
Mg2—O2^vi	1.9845 (1)	O3—Mg2^xii	1.9030 (1)
Mg2—O3^vii	1.9030 (1)	O3—P1	1.5806 (1)
Mg2—O5	2.0661 (1)	O4—Mg1^xiii	2.0499 (1)
Mg2—O6^viii	2.0939 (1)	O4—Mg1^iv	2.2412 (1)
Mg2—O7^ix	2.3814 (1)	O4—P1	1.6013 (1)
P1—O1	1.7854 (1)	O5—Mg2	2.0661 (1)
P1—O3	1.5806 (1)	O5—P1	1.5425 (1)
P1—O4	1.6013 (1)	O6—Mg1	1.8503 (1)
P1—O5	1.5425 (1)	O6—Mg2^i	2.0939 (1)
P2—O1	1.4870 (1)	O6—P2	1.6091 (1)
P2—O2	1.5021 (1)	O7—Mg1^xiv	1.9461 (1)
P2—O6	1.6091 (1)	O7—Mg2^ix	2.3814 (1)
P2—O7	1.6040 (1)	O7—P2	1.6040 (1)
O2^xv—Mg1—O3^xvi	78.680 (4)	O1—P1—O5	115.771 (2)
O2^xv—Mg1—O4^iii	84.872 (3)	O3—P1—O4	103.426 (3)
O2^xv—Mg1—O4^iv	154.7676 (11)	O3—P1—O5	112.5380 (13)
O2^xv—Mg1—O6	85.052 (3)	O4—P1—O5	115.496 (4)
O2^xv—Mg1—O7^v	101.344 (4)	O1—P2—O2	107.107 (5)
O3^xvi—Mg1—O4^iii	90.507 (3)	O1—P2—O6	103.548 (3)
O3^xvi—Mg1—O4^iv	94.190 (4)	O1—P2—O7	99.856 (3)
O3^xvi—Mg1—O6	82.185 (3)	O2—P2—O6	118.856 (3)
O3^xvi—Mg1—O7^v	171.0005 (5)	O2—P2—O7	116.6010 (19)
O4^iii—Mg1—O4^iv	70.899 (3)	O6—P2—O7	108.259 (4)
O4^iii—Mg1—O6	168.5470 (8)	P1—O1—P2	140.125 (3)
O4^iii—Mg1—O7^v	98.467 (3)	Mg1^xv—O2—Mg2^x	97.738 (4)
O4^iv—Mg1—O6	118.245 (3)	Mg1^xviii—O2—P2	133.3317 (16)
O4^iv—Mg1—O7^v	89.414 (4)	Mg2^x—O2—P2	127.599 (2)
O6—Mg1—O7^v	88.844 (3)	Mg1^xix—O3—Mg2^xii	97.926 (4)
O2^vi—Mg2—O3^vii	83.208 (4)	Mg1^xx—O3—P1	127.0411 (19)
O2^vi—Mg2—O5	146.6086 (16)	Mg2^xii—O3—P1	135.016 (2)
O2^vi—Mg2—O6^xvii	98.094 (4)	Mg1^xiii—O4—Mg1^iv	109.101 (3)
O2^vi—Mg2—O7^ix	95.548 (4)	Mg1^xiii—O4—P1	131.173 (2)
O3^vii—Mg2—O5	86.937 (4)	Mg1^iv—O4—P1	115.787 (3)
O3^vii—Mg2—O6^xvii	170.2380 (6)	Mg2—O5—P1	141.9393 (17)
O3^vii—Mg2—O7^ix	97.705 (4)	Mg1—O6—Mg2^xxi	105.681 (3)
O5—Mg2—O6^xvii	96.931 (4)	Mg1—O6—P2	135.320 (2)
O5—Mg2—O7^ix	117.404 (3)	Mg2^xxii—O6—P2	118.845 (3)
O6^xvii—Mg2—O7^ix	72.552 (3)	Mg1^xiv—O7—Mg2^ix	92.744 (3)
O1—P1—O3	100.385 (5)	Mg1^xiv—O7—P2	167.7228 (6)
O1—P1—O4	107.510 (3)	Mg2^ix—O7—P2	96.943 (3)

Symmetry codes: (i) x, −y+3/2, z−1/2; (ii) x−1, −y+3/2, z−1/2; (iii) x−1, y, z−1; (iv) −x+1, −y+2, −z+1; (v) −x+1, y+1/2, −z+1/2; (vi) −x+1, y−1/2, −z+3/2; (vii) −x+2, y−1/2, −z+3/2; (viii) x, −y+3/2, z+1/2; (ix) −x+1, −y+1, −z+1; (x) −x+1, y+1/2, −z+3/2; (xi) x+1, −y+3/2, z+1/2; (xii) −x+2, y+1/2, −z+3/2; (xiii) x+1, y, z+1; (xiv) −x+1, y−1/2, −z+1/2; (xv) x, −y+5/2, z+1/2; (xvi) x−1, −y+5/2, z+1/2; (xvii) x, −y+5/2, z+3/2; (xviii) x−1, −y+3/2, z+3/2; (xix) x+1, −y+5/2, z+1/2; (xx) x, −y+3/2, z+3/2; (xxi) x, −y+5/2, z−1/2; (xxii) x−1, −y+3/2, z+1/2.

(phase_3) trimagnesium ocyaoxidodiphosphate

Crystal data

Mg3P2O8	Z = 2
Mr = 262.87	F(000) = 260.0
Monoclinic, P21/n	Dx = 2.745 Mg m^−3
Hall symbol: -P 2yn	Cu Kα1 radiation, λ = 1.5406 Å
a = 7.594 (3) Å	T = 296 K
b = 8.282 (8) Å	white
c = 5.071 (4) Å	flat sheet, 20 × 20 mm
β = 94.01 (6)°	Specimen preparation: Prepared at 296 K
V = 318.1 (4) Å^3

Data collection

Bruker D8 Advance diffractometer	Data collection mode: reflection
Radiation source: sealed X-ray tube, Bruker Cu Ceramic X-ray tube	Scan method: step
Ge 111 monochromator	2θmin = 8.0°, 2θmax = 140.422°, 2θstep = 0.017°
Specimen mounting: packed powder pellet

Refinement

Least-squares matrix: full	7939 data points
Rp = 0.048	Profile function: CW Profile function number 4 with 21 terms Pseudovoigt profile coefficients as parameterized in Thompson et al. [Thompson, P., Cox, D. E. & Hastings, J. B. (1987). J. Appl. Cryst. 20, 79–83]. Asymmetry correction of Finger et al. [Finger, L. W., Cox, D. E. & Jephcoat, A. P. (1994). J. Appl. Cryst. 27, 892–900]. Microstrain broadening by Stephens [Stephens, P. W. (1999). J. Appl. Cryst. 32, 281–289]. #1(GU) = 0.000 #2(GV) = 61.116 #3(GW) = -3.576 #4(GP) = 8.238 #5(LX) = 9.783 #6(ptec) = 0.00 #7(trns) = 0.00 #8(shft) = 3.1799 #9(sfec) = 0.00 #10(S/L) = 0.0005 #11(H/L) = 0.0005 #12(eta) = 0.7500 #13(S400 ) = 0.0E+00 #14(S040 ) = 0.0E+00 #15(S004 ) = 0.0E+00 #16(S220 ) = 0.0E+00 #17(S202 ) = 0.0E+00 #18(S022 ) = 0.0E+00 #19(S301 ) = 0.0E+00 #20(S103 ) = 0.0E+00 #21(S121 ) = 0.0E+00 Peak tails are ignored where the intensity is below 0.0020 times the peak Aniso. broadening axis 0.0 0.0 1.0
Rwp = 0.063	81 parameters
Rexp = 0.025	0 restraints
RBragg = 0.047	Weighting scheme based on measured s.u.'s
R(F) = 0.049	(Δ/σ)max = 0.02
R(F^2) = 0.04372	Background function: GSAS Background function number 1 with 36 terms. Shifted Chebyshev function of 1st kind 1: 667.815 2: -121.026 3: 250.928 4: -108.029 5: 130.095 6: -120.393 7: 81.5238 8: -66.4354 9: 31.4946 10: -19.2674 11: 16.0150 12: -21.1906 13: 24.6787 14: -7.73173 15: 5.82669 16: 7.16351 17: 9.66948 18: 8.02466 19: 5.65618 20: 4.74674 21: -11.8793 22: -0.873263 23: -2.40368 24: -9.55052 25: -6.36605 26: 6.28370 27: -8.07052 28: 6.33955 29: -12.6008 30: 3.68005 31: 5.95000 32: 7.62516 33: 1.58465 34: 5.63269 35: -2.90398 36: 14.7130
χ^2 = 6.250

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Mg1	0.6095	0.1432	0.092	0.015*
Mg2	0.0	0.0	0.5	0.015*
P1	0.1996	0.1946	0.0355	0.015*
O1	0.0589	0.1446	0.8188	0.015*
O2	0.1262	0.1995	0.3036	0.015*
O3	0.2592	0.3629	0.947	0.015*
O4	0.3545	0.0759	0.0459	0.015*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
?	?	?	?	?	?	?

Geometric parameters (Å, º)

Mg1—O1^i	2.1496 (14)	P1—O4	1.5310 (7)
Mg1—O2^i	1.9692 (11)	O1—Mg1^vii	2.1496 (14)
Mg1—O3^i	2.0613 (16)	O1—Mg2	2.0363 (12)
Mg1—O4	2.0128 (9)	O1—P1^viii	1.5350 (11)
Mg1—O4^ii	1.9706 (16)	O2—Mg1^vii	1.9692 (11)
Mg2—O1	2.0363 (12)	O2—Mg2	2.1849 (13)
Mg2—O1^iii	2.0363 (12)	O2—P1	1.5055 (11)
Mg2—O2	2.1849 (13)	O3—Mg1^vii	2.0613 (16)
Mg2—O2^iii	2.1849 (13)	O3—Mg2^ix	2.1532 (9)
Mg2—O3^iv	2.1532 (9)	O3—P1^viii	1.5419 (12)
Mg2—O3^v	2.1532 (9)	O4—Mg1	2.0128 (9)
P1—O1^vi	1.5350 (11)	O4—Mg1^ii	1.9706 (16)
P1—O2	1.5055 (11)	O4—P1	1.5310 (7)
P1—O3^vi	1.5419 (12)
O1^i—Mg1—O2^i	83.09 (6)	O2^iii—Mg2—O3^iv	86.56 (5)
O1^i—Mg1—O3^i	70.19 (3)	O2^iii—Mg2—O3^v	93.44 (5)
O1^x—Mg1—O4	94.70 (4)	O3^iv—Mg2—O3^v	180.0
O1^x—Mg1—O4^ii	167.723 (8)	O1^vi—P1—O2	111.75 (5)
O2^i—Mg1—O3^i	127.20 (3)	O1^vi—P1—O3^vi	103.84 (3)
O2^x—Mg1—O4	102.03 (5)	O1^vi—P1—O4	110.24 (4)
O2^x—Mg1—O4^ii	109.14 (6)	O2—P1—O3^vi	111.91 (3)
O3^x—Mg1—O4	124.19 (5)	O2—P1—O4	108.56 (4)
O3^x—Mg1—O4^ii	101.87 (2)	O3^vi—P1—O4	110.49 (5)
O4—Mg1—O4^ii	81.95 (3)	Mg1^xi—O1—Mg2	95.37 (6)
O1—Mg2—O1^iii	180.0	Mg1^xii—O1—P1^viii	91.20 (4)
O1—Mg2—O2	80.69 (6)	Mg2—O1—P1^viii	146.12 (2)
O1—Mg2—O2^iii	99.31 (6)	Mg1^xi—O2—Mg2	96.24 (6)
O1—Mg2—O3^iv	94.22 (6)	Mg1^xii—O2—P1	137.36 (3)
O1—Mg2—O3^v	85.78 (6)	Mg2—O2—P1	126.38 (3)
O1^iii—Mg2—O2	99.31 (6)	Mg1^xi—O3—Mg2^ix	122.25 (4)
O1^iii—Mg2—O2^iii	80.69 (6)	Mg1^xii—O3—P1^viii	94.41 (2)
O1^iii—Mg2—O3^iv	85.78 (6)	Mg2^ix—O3—P1^viii	134.78 (3)
O1^iii—Mg2—O3^v	94.22 (6)	Mg1—O4—Mg1^ii	98.05 (3)
O2—Mg2—O2^iii	179.9802	Mg1—O4—P1	123.83 (4)
O2—Mg2—O3^iv	93.44 (5)	Mg1^ii—O4—P1	134.78 (3)
O2—Mg2—O3^v	86.56 (5)

Symmetry codes: (i) x+1/2, −y+1/2, z−1/2; (ii) −x+1, −y, −z; (iii) −x, −y, −z+1; (iv) −x+1/2, y−1/2, −z+3/2; (v) x−1/2, −y+1/2, z−1/2; (vi) x, y, z−1; (vii) x−1/2, −y+1/2, z+1/2; (viii) x, y, z+1; (ix) −x+1/2, y+1/2, −z+3/2; (x) x+3/2, −y+3/2, z+1/2; (xi) x+1/2, −y+3/2, z+1/2; (xii) x−1/2, −y+1/2, z+3/2.