KSbO(Ge_0.32Si_0.68)O₄, a KTP isomorph

A structural model of potassium antimony germanate/silicate (0.32/0.68), KSbO(Ge_0.32Si_0.68)O₄, has been determined at room temperature. KSbO(Ge_0.32Si_0.68)O₄ belongs to the KTiOPO₄ (KTP) isomorphic family and is composed of SbO₆ octahedra (site symmetry and 2) arranged in helical chains bridged by (Ge/Si)O₄ tetrahedra. Germanium and silicon have a similar distribution in the crystallographically independent tetrahedra (site symmetry 2). The structure contains large cavities occupied by the K atom. Two partially occupied potassium positions have been identified 1.273 (8) Å apart, with an indication of a third potassium position between them. At room temperature, KSbO(Ge_0.32Si_0.68)O₄ crystallizes in the paraelectric phase of space group Pnan. This phase is found at elevated temperatures for almost all KTiOPO₄ isomorphic compounds and KSbO(Ge_0.32Si_0.68)O₄ is the second isomorph that is paraelectric at room temperature.

S1. Comment

Since the structure determination by Tordjman et al. (1974) of potassium titanyl phosphate, KTiOPO₄ (KTP), it has evolved to a well known material for use in nonlinear optical processes (Zumsteg et al., 1976), such as second harmonic generation (SHG) (Bierlein et al., 1989). The high nonlinearity of KTP extends to many isomorphic compounds with the general chemical formula AMOBO₄ (A = K, Rb, Cs or Tl, M = Ti and B = P or As). Several of these isomorphs have been extensively studied (Stucky et al., 1989, and references therein). Recently another group of isomorphs have been explored, with substitutions in the MO₆ octahedra and the BO₄ tetrahedra (A = Na, K, Rb, Tl or Ag, M = Sb, Nb or Ta and B = Si or Ge). The isomorphs with Sb in the octahedra are fairly well studied. KSbOSiO₄ (KSS) was prepared by Crosnier et al. (1990) in a solid state reaction and the structure of KSbOGeO₄ (KSG) was published independently by Pagnoux et al. (1991) and Belokoneva et al. (1991). Other compounds include NaSbOSiO₄ (Pagnoux et al., 1992), RbSbOGeO₄ (Favard et al., 1992), TlSbOGeO₄ (Belokoneva & Mill, 1992a), RbSbOGeO₄ (Belokoneva & Mill, 1992b), NaSbOGeO₄, AgSbOGeO₄, AgSbOSiO₄ (Mill et al., 1993).

The KTP isomorphic materials with Sb in the oxygen octahedra have all weak nonlinear optical coefficients, which are orders of magnitudes lower than those of KTP. This is mainly due to the low polarizability of Sb^V and the regularity of the SbO₆ octahedra. Butashin et al. (1994) reported that an exchange of Sb^V with Nb^V greatly increases the nonlinear optical coefficients. The solid solution system of KSb_{1 - x}Nb_xOGeO₄ extends to x = 3/4, but no KTP phase has been detected in the A₂O—Nb₂O₅—BO₂ (A = Na, K, Rb, Tl or Ag and B = Si or Ge) system (Belekoneva et al., 1991).

As a part of our ongoing investigation of new modifications and isomorphic materials in the KTP family, we have prepared a new member of the Sb family with mixed occupation of Ge and Si in the tetrahedral positions, KSbOGe_0.32Si_0.68O₄ (KSGS).

The structure of KSGS is very similar to the KTP structure (Thomas et al., 1990). It comprises SbO₆ octahedra that are corner linked at alternating cis and trans positions, forming helical chains in the [011] direction. These chains are further linked by (Ge/Si)O₄ tetrahedra. The framework built by the octahedra and tetrahedra has large cavities which are occupied by the K cations. Fig. 1 gives a view of the structure in the (010) plane.

All SbO₆ octahedra are fairly regular with maximum and minimum Sb—O bond lengths of 2.004 (5) Å and 1.955 (6) Å, respectively, with a mean (standard deviation) of 1.97 (2) Å. The AO₄ tetrahedra (A = Ge,Si) are also regular with maximum and minimum bond lengths of 1.683 (6) Å and 1.648 (6) Å and a mean (st. d) of 1.664 (15) Å. The mean (st. d) Si—O bond length of the SiO₄ tetrahedra in KSS is 1.62 (2) Å (Crosnier et al., 1990) while that of the GeO₄ tetrahedra in KSG is 1.742 (14) Å (Belokoneva et al., 1991). It should also be noted that the shape and bond lengths of the SbO₆ octahedra are almost identical in KSS, KSG and the new KSGS. There is however one large difference between KSGS and the KSS and KSG structures, in that KSGS has split cation positions. The K and K' positions are separated by 1.273 (8) Å, each with an occupancy of about 0.50. Split cation positions have been reported earlier for the ASbOGeO₄ materials in the high-temperature paraelectric phase but with shorter distances between the split positions, e.g. 0.561 (5) Å for TlSbOGeO₄ at 293 K (Belokoneva & Mill, 1992a) and 0.703 (13) Å for RbSbOGeO₄ at 503 K (Belokoneva et al., 1997).

The coordination sphere of oxygen atoms around K and K' are irregular. K is coordinated by six O atoms, resulting in four short bonds [2.707 (8) < K—O < 2.866 (8) Å] and two longer interactions up to 3.112 (7) Å. K' has seven coordinating O atoms, four shorter bonds [2.659 (8) < K'—O < 2.734 Å] and three longer interactions up to 3.146 (7) Å. Table 1 gives all bond lengths for K and K'. The four short K—O bonds at each split position (< 2.90 Å) are approximately in the (001) plane. A similar trend of irregular coordination around the potassium sites exist in KTP (Thomas et al., 1990) but with eight coordinating O atoms for K1 with interactions up to 3.057 (3) Å and nine coordinating O atoms for K2 within 3.117 (3) Å. Table 2 gives the unit-cell parameters for KSS, KSG and KSGS.

The high temperature modifications (space group Pnan) of KSS and KSG were studied by Farvard et al. (1994) at temperatures of T_c + 30 K. The distance between the cation split positions were 0.909 (13) in KSS and 0.843 (13) in KSG. It was noted that the deviation from centrosymmetry in the low-temperature phase of Pna2₁ was mainly due to the location of A⁺ ions and not from the framework. The changes in the framework during the phase transition are small compared to KTP, mainly due to the regularity of the SbO₆ compared to the TiO₆ octahedra in which the Ti^IV atoms are off-centre. The phase transition is displasive for the framework and of order-disorder type for the cations (Farvard et al., 1994). The centrosymmetry of ASbO(Ge,Si)O₄ compounds arises from the partial but equal occupation of two non-equivalent cation sites, resulting in the paraelectric phase. At low temperature the four partly occupied A⁺ sites reduces to two fully occupied sites, resulting in an ordered structure with Pna2₁ symmetry.

All of those previously studied Sb containing materials are isomorphic to KTP with a ferroelectric to paraelectric phase transition well above room temperature (from space group Pna2₁ to Pnan). An exception is TlSbOGeO₄ with a T_c of 272 K (Stefanovich et al., 1993). The T_c of both KSS and KSG was determined as 600 K by Stefanovich et al. (1993). KSGS is the second material in the KTP family to have a T_c below room temperature. Preliminary investigations of KSGS at low temperatures (173 and 113 K) show that the space group still is Pnan, i.e., the value of T_c must be very low.

Related literature

For related literature, see: Belokoneva & Mill (1992a, 1992b); Belokoneva et al. (1991, 1997); Bierlein et al. (1989); Bolt & Bennema (1990); Butashin et al. (1994); Crosnier et al. (1990); Farvard et al. (1994); Hong (1974); Larson (1970); Mill et al. (1993); Pagnoux et al. (1991, 1992); Stefanovich et al. (1993); Stucky et al. (1989); Thomas et al. (1990); Tordjman et al. (1974); Wilson (1949); Zachariasen (1967); Zumsteg et al. (1976).

S2. Experimental

The crystals were obtained in a platinum crucible by spontaneous crystallization in a PbO₂-flux containing equimolar amounts of K₂CO₃, Sb₂O₅, GeO₂ and SiO₂ carefully mixed together. The weight ratio between PbO₂ and the crystallization material was 1:1. The mixture was slowly heated for 4 days to 1273 K in order to obtain a homogeneous melt and the temperature then decreased to 1023 K at 1.4 K h^-1. The brown coloured flux was later dissolved in 5 M HCl. Most of the flux dissolved within 24 h. The small fine crystals in the resulting yellow powder had the typical KTP morphology (Bolt & Bennema, 1990).

It may also be noted that the mixing of K₂CO₃, Sb₂O₅, GeO₂ and SiO₂ without any PbO₂ as a flux resulted in a powder (no melting occurred at 1273 K) containing small fine crystals of K₃Sb₅O₁₄, whose structure has been published by Hong (1974).

S3. Refinement

Wilson (1949) statistics clearly indicate a centrosymmetric structure. However, an attempt to refine the structure in space group Pna2₁, using the coordinates of KTP (Thomas et al. 1990) as the starting set led to severe correlation problems. No indication of any twinning was found in the measured KSGS crystal. Independent measurement of three different KSGS crystals from the same batch have each resulted in the same space group (Pnan). Careful examination of the reflection profiles of our CAD-4 diffractometer data shows nothing out of the ordinary and no extra peaks appear using an area detector.

The highest residual electron density peak (based on the observed reflections) have a Δρ of 3.410 e Å^-3 and is located between the K and K' positions while the lowest peak with a Δρ of -3.921 e Å^-3 is 1.571 (6) Å from the K position. Δρ maps of the area around and between K and K' shows a single larger peak positioned 0.655 (6) Å from K and 0.643 (6) Å from K'. The peak could be refined as a possible K'' site, to an occupancy factor of 0.07 using restraints. Because of the restraints we have chosen to neglect K'' and report the structure with two split K positions as our final structural model.

An isotropic extinction parameter (Zachariasen, 1967) was refined using Larson's implementation (Larson, 1970). About 14% of the reflections were affected with a maximum correction of y = 0.90 for the 022 reflection (the observed structure factor is F_obs = yF_kin, were F_kin is the kinematic value of the structure factor).

Computing details

Data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: LATCON in Xtal3.7 (Hall et al., 2000); data reduction: DIFDAT, SORTRF, ADDREF and ABSORB in Xtal3.7; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: CRILSQ in Xtal3.7; molecular graphics: ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: BONDLA, ATABLE and CIFIO in Xtal3.7.

Figures

Fig. 1. ORTEPIII (Burnett & Johnson, 1996) view of the KSbOGe0.32Si0.68O4 structure in the (010) plane with the possible K3 position as indicated by Δρ maps. Displacement ellipsoids are drawn at the 80% probability level. [Symmetry codes: (i) 1 - x, y + 1/2, z + 1/2; (ii) 1 - x, y - 1/2, z + 1/2; (iii) -x + 3/2, -y + 3/2, z + 1/2; (iv) 1 - x, 2 - y, 1 - z; (v) 1 - x, 1 - y, 1 - z; (vi) -x + 3/2, y, 1 - z; (vii) x, -y + 3/2, -z + 1/2; (viii) 1 - x, y + 1/2, z - 1/2.]

(KSb)

Crystal data

KSbOGe0.32Si0.68O4	F(000) = 1038
Mr = 283.06	Dx = 4.118 Mg m^−3
Orthorhombic, Pnan	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2n 2bc	Cell parameters from 21 reflections
a = 13.0970 (9) Å	θ = 35.1–39.6°
b = 6.5310 (5) Å	µ = 9.13 mm^−1
c = 10.6873 (6) Å	T = 293 K
V = 914.15 (11) Å^3	Prism, colourless
Z = 8	0.06 × 0.05 × 0.04 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	1585 reflections with F^2 > 2σ(F^2)
Radiation source: Fine-focus sealed tube	Rint = 0.074
Graphite monochromator	θmax = 39.8°, θmin = 3.1°
ω–2θ scans	h = 0→23
Absorption correction: analytical (Alcock, 1974)	k = 0→11
Tmin = 0.784, Tmax = 0.804	l = −19→19
6155 measured reflections	3 standard reflections every 240 min
2818 independent reflections	intensity decay: none

Refinement

Refinement on F^2	13 constraints
Least-squares matrix: full	w = 1/(σ^2(Fsqd) + 0.010(Fsqd)^2)
R[F^2 > 2σ(F^2)] = 0.046	(Δ/σ)max = 0.39 × 10 -3
wR(F^2) = 0.168	Δρmax = 3.41 e Å^−3
S = 1.00	Δρmin = −3.92 e Å^−3
2818 reflections	Extinction correction: Gaussian (Zachariasen, 1967), Eq22 p292 "Cryst. Comp." Munksgaard 1970
89 parameters	Extinction coefficient: 1.9 (9) × 10^3
0 restraints

Special details

Refinement. The following constraints were used during the refinement: H-atom parameters constrained pop(K')=1.0–1.0*pop(K) H-atom parameters constrained y(Si1)=0.0 + 1.0*y(Ge1) H-atom parameters constrained u11(Si1)=0.0 + 1.0*u11(Ge1) H-atom parameters constrained u22(Si1)=0.0 + 1.0*u22(Ge1) H-atom parameters constrained u33(Si1)=0.0 + 1.0*u33(Ge1) H-atom parameters constrained u13(Si1)=0.0 + 1.0*u13(Ge1) H-atom parameters constrained pop(Si1)=1.0–1.0*pop(Ge1) H-atom parameters constrained x(Si2)=0.0 + 1.0*x(Ge2) H-atom parameters constrained u11(Si2)=0.0 + 1.0*u11(Ge2) H-atom parameters constrained u22(Si2)=0.0 + 1.0*u22(Ge2) H-atom parameters constrained u33(Si2)=0.0 + 1.0*u33(Ge2) H-atom parameters constrained u23(Si2)=0.0 + 1.0*u23(Ge2) H-atom parameters constrained pop(Si2)=1.0–1.0*pop(Ge2)

No restraints used.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
K	0.8594 (3)	1.0507 (7)	0.3188 (5)	0.022 (2)	0.491 (11)
K'	0.8683 (4)	1.0254 (6)	0.4364 (6)	0.024 (2)	0.508 (11)
Sb1	0.63209 (4)	0.75	0.25	0.0050 (2)
Sb2	0.50	0.50	0.50	0.0057 (2)
Ge1	0.75	0.5722 (3)	0.50	0.0060 (8)	0.296 (11)
Si1	0.75	0.5722 (3)	0.50	0.0060 (8)	0.703 (11)
Ge2	1.06956 (13)	0.75	0.25	0.0059 (8)	0.352 (10)
Si2	1.06956 (13)	0.75	0.25	0.0059 (8)	0.647 (10)
O1	0.7395 (5)	0.7265 (10)	0.3762 (6)	0.015 (3)
O2	0.6480 (4)	0.4270 (10)	0.5165 (6)	0.011 (3)
O3	0.4878 (5)	0.2926 (8)	0.3687 (5)	0.009 (2)
O4	0.6428 (5)	1.0497 (9)	0.2802 (6)	0.012 (3)
O5	0.4739 (4)	0.2820 (10)	0.6225 (6)	0.012 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
K	0.0143 (17)	0.0179 (18)	0.035 (2)	−0.0077 (13)	0.0010 (15)	−0.0012 (16)
K'	0.024 (2)	0.0103 (16)	0.037 (3)	0.0048 (13)	−0.0068 (17)	−0.0032 (14)
Sb1	0.0047 (2)	0.0054 (2)	0.0048 (2)	0.00000	0.00000	0.0009 (2)
Sb2	0.0066 (2)	0.0059 (2)	0.0045 (2)	−0.0009 (2)	0.00137 (18)	−0.0003 (2)
Ge1	0.0048 (7)	0.0071 (8)	0.0060 (7)	0.00150	−0.0005 (6)	0.00150
Si1	0.0048 (7)	0.0071 (8)	0.0060 (7)	0.00150	−0.0005 (6)	0.00150
Ge2	0.0067 (7)	0.0059 (7)	0.0052 (7)	0.00140	0.00140	0.0008 (6)
Si2	0.0067 (7)	0.0059 (7)	0.0052 (7)	0.00140	0.00140	0.0008 (6)
O1	0.011 (2)	0.017 (3)	0.017 (2)	−0.006 (2)	−0.0067 (19)	0.010 (2)
O2	0.005 (2)	0.014 (2)	0.013 (3)	0.0005 (17)	−0.0006 (17)	−0.002 (2)
O3	0.013 (2)	0.006 (2)	0.009 (2)	−0.0051 (17)	0.0047 (17)	−0.0045 (15)
O4	0.018 (3)	0.006 (2)	0.013 (2)	−0.001 (2)	−0.0019 (18)	0.0005 (17)
O5	0.010 (2)	0.014 (3)	0.011 (2)	−0.0016 (18)	0.0053 (17)	0.003 (2)

Geometric parameters (Å, º)

K—K'	1.273 (8)	Sb2—O3	1.957 (6)
K—O1	2.707 (8)	Sb2—O5	1.964 (6)
K—O5	2.728 (7)	Sb2—O2	2.004 (5)
K—O3	2.854 (7)	Sb2—O3	1.957 (6)
K—O4	2.866 (8)	Sb2—O5	1.964 (6)
K—O2	3.024 (7)	Ge1—O1	1.669 (6)
K—O3	3.112 (7)	Ge1—O2	1.648 (6)
K'—O1	2.659 (8)	Ge1—O1	1.669 (6)
K'—O2	2.679 (7)	Ge1—O2	1.648 (6)
K'—O3	2.700 (7)	Si1—O1	1.669 (6)
K'—O5	2.734 (8)	Si1—O2	1.648 (6)
K'—O4	3.036 (8)	Si1—O1	1.669 (6)
K'—O1	3.133 (8)	Si1—O2	1.648 (6)
K'—O5	3.146 (7)	Ge2—O4	1.654 (6)
Sb1—O1	1.955 (6)	Ge2—O3^i	1.683 (6)
Sb1—O4	1.989 (6)	Ge2—O3	1.683 (6)
Sb1—O1	1.955 (6)	Ge2—O4	1.654 (6)
Sb1—O4	1.989 (6)	Si2—O4	1.654 (6)
Sb1—O5	1.957 (6)	Si2—O3^i	1.683 (6)
Sb1—O5	1.957 (6)	Si2—O3	1.683 (6)
Sb2—O2	2.004 (5)	Si2—O4	1.654 (6)
K'—K—O1	74.2 (3)	O3—Sb2—O5	87.8 (2)
K'—K—O4	103.4 (4)	O1—Ge1—O2	111.4 (3)
K'—K—K	126.8 (4)	O1—Ge1—O1	105.7 (3)
K'—K—O5	76.8 (3)	O1—Ge1—O2	109.2 (3)
K'—K—O3	70.1 (3)	O2—Ge1—O1	109.2 (3)
O1—K—O4	57.1 (2)	O2—Ge1—O2	109.7 (3)
O1—K—K	142.4 (2)	O1—Ge1—O2	111.4 (3)
O1—K—O5	147.7 (3)	O1—Si1—O2	111.4 (3)
O1—K—O3	71.6 (2)	O1—Si1—O1	105.7 (3)
O4—K—K	86.1 (2)	O1—Si1—O2	109.2 (3)
O4—K—O5	145.7 (3)	O2—Si1—O1	109.2 (3)
O4—K—O3	127.5 (2)	O2—Si1—O2	109.7 (3)
K—K—O5	68.35 (19)	O1—Si1—O2	111.4 (3)
K—K—O3	140.9 (2)	O4—Ge2—O3^i	110.7 (3)
O5—K—O3	85.4 (2)	O4—Ge2—O3	112.7 (3)
K—K'—O1	78.4 (3)	O4—Ge2—O4	109.1 (3)
K—K'—O2	92.9 (3)	O3^i—Ge2—O3	101.0 (3)
K—K'—O5	76.2 (3)	O3^i—Ge2—O4	112.7 (3)
K—K'—O3	83.6 (3)	O3—Ge2—O4	110.7 (3)
O1—K'—O2	135.5 (3)	O4—Si2—O3^i	110.7 (3)
O1—K'—O5	150.9 (3)	O4—Si2—O3	112.7 (3)
O1—K'—O3	74.8 (2)	O4—Si2—O4	109.1 (3)
O2—K'—O5	60.2 (2)	O3^i—Si2—O3	101.0 (3)
O2—K'—O3	148.2 (3)	O3^i—Si2—O4	112.7 (3)
O5—K'—O3	88.4 (2)	O3—Si2—O4	110.7 (3)
O1—Sb1—O4	85.1 (3)	K—O1—K'	27.43 (18)
O1—Sb1—O1	88.0 (2)	K—O1—Sb1	101.5 (3)
O1—Sb1—O4	89.1 (3)	K—O1—Ge1	127.2 (3)
O1—Sb1—O5	91.2 (2)	K—O1—Si1	127.2 (3)
O1—Sb1—O5	178.2 (3)	K'—O1—Sb1	124.5 (3)
O4—Sb1—O1	89.1 (3)	K'—O1—Ge1	101.5 (3)
O4—Sb1—O4	171.9 (3)	K'—O1—Si1	101.5 (3)
O4—Sb1—O5	92.4 (3)	Sb1—O1—Ge1	130.8 (4)
O4—Sb1—O5	93.3 (3)	Sb1—O1—Si1	130.8 (4)
O1—Sb1—O4	85.1 (3)	Sb2—O2—Ge1	129.6 (4)
O1—Sb1—O5	178.2 (3)	Sb2—O2—Si1	129.6 (4)
O1—Sb1—O5	91.2 (2)	Sb2—O2—K'	100.0 (2)
O4—Sb1—O5	93.3 (3)	Ge1—O2—K'	130.4 (3)
O4—Sb1—O5	92.4 (3)	Si1—O2—K'	130.4 (3)
O5—Sb1—O5	89.6 (2)	Sb2—O3—Ge2	127.1 (3)
O2—Sb2—O3	88.7 (2)	Sb2—O3—K	136.5 (3)
O2—Sb2—O5	86.4 (2)	Sb2—O3—K'	112.8 (3)
O2—Sb2—O2	180	Ge2—O3—K	96.0 (3)
O2—Sb2—O3	91.3 (2)	Ge2—O3—K'	116.3 (3)
O2—Sb2—O5	93.6 (2)	K—O3—K'	26.32 (18)
O3—Sb2—O5	87.8 (2)	K—O4—Sb1	95.5 (2)
O3—Sb2—O2	91.3 (2)	K—O4—Ge2	126.9 (3)
O3—Sb2—O3	180	Sb1—O4—Ge2	134.9 (4)
O3—Sb2—O5	92.2 (2)	Sb2—O5—K	114.6 (3)
O5—Sb2—O2	93.6 (2)	Sb2—O5—K'	99.2 (2)
O5—Sb2—O3	92.2 (2)	Sb2—O5—Sb1	131.7 (3)
O5—Sb2—O5	180	K—O5—K2	26.96 (18)
O2—Sb2—O3	88.7 (2)	K—O5—Sb1	110.4 (3)
O2—Sb2—O5	86.4 (2)	K'—O5—Sb1	129.1 (3)

Symmetry code: (i) x+1/2, y+1/2, −z+1/2.