MgNB₉, a new magnesium nitridoboride

The structure of a new magnesium nitridoboride, MgNB₉, has been refined from single-crystal X-ray data. The Mg and N atoms lie on sites with crystallographic 3m symmetry. The structure consists of two layers alternating along the c axis. The NB₆ layer, with B₁₂ icosahedra, has the C₂B₁₃ structure type. Within this layer, boron icosahedra are bonded to N atoms, each coordinating to three boron polyhedra. Another MgB₃ layer, with B₆ octahedra, does not belong to any known structure type. The boron icosahedra and octahedra are connected to each other, thus forming a three-dimensional boron framework.

S1. Comment

The discovery of superconductivity at T_c ≈ 39 K in MgB₂ (Nagamatsu et al., 2001) has attracted great scientific and applications interest. Single crystals are indispensable for studies of the physical properties of this compound. During attempts to grow single crystals of MgB₂ in a BN crucible, black hexagonal prism crystals were obtained. Energy-dispersive X-ray diffraction analysis showed the presence of Mg, B and N. Single-crystal analysis was performed to determine the structure and composition of this new phase, MgNB₉, and the results are presented here.

The structure of MgNB₉ is presented in Fig. 1. It may be considered as an intergrowth of two layers, NB₆ (Fig. 2) and MgB₃ (Fig. 3a). Boron polyhedra from different layers are connected to each other, thus forming a rigid three-dimensional framework with channels along the [110] direction, in which Mg and N atoms are located. Probably for this reason, the atomic displacement parameters for the B and N atoms are very small (U_ii \sim 0.0035 Ų), especially compared with those of Mg (U₁₁ = 0.013 Ų and U₃₃ = 0.006 Ų).

The NB₆ layer is similar to the CB₆ layer in the structure of C₂B₁₃ (Will & Kossobutzki, 1976). It is built of boron icosahedra which are connected to each other within the layer through the N atoms, and each N atom is bonded to three boron polyhedra, with a B—N—B angle of 115.22°(3). Each B atom in this layer forms five bonds within the icosahedron, and one bond with either an N atom (B1) or a B atom from the boron octahedron of another layer (B2). All B—B distances are typical for this type of compound.

It is not of value to compare known B—N distances with those of MgNB₉. Usually, boron nitrides are obtained in nitrogen-rich regions and have quite different mutual coordination of the B and N atoms. For the most part, B—N bonds in boron nitrides are much shorter (1.34–1.45 Å) than in MgNB₉ (1.524 Å), but in some cases may go as high as 1.50 Å in UBN (Klesnar & Rogl, 1991) and 1.54 Å in PrBN₂ (Rogl & Klesnar, 1992).

Two neighbouring N atoms are bonded to Mg atoms from different layers and are shifted from the average layer plane in different directions. Mg—N distances in nitrides and boron nitrides vary in a wide range from 1.91 to 2.18 Å, but the most reliable data give the range 2.03–2.15 Å for both nitrides (Mg₃N₂; Partin et al., 1997) and boron nitrides (Mg₃BN₃; Hiraguchi et al., 1991), which is about the same as found in the present study (Table 1).

A structural type for the other layer is unknown to date, although structures with a boron octahedron framework do exist. The CaB₆ structural type (von Stackelberg & Neumann, 1932) may be taken as a starting point if we consider its octahedral layer perpendicular to the body diagonal of the cubic unit cell (Fig. 3 b), but the orientations of the octahedra are different: in MgNB₉, they are rotated 30° around the 3 axis of the hypothetical cubic unit cell compared with the CaB₆ structure. Structural transformations are known for borides. In the structure of Li₂B₆ (von Schnering et al., 1999), the boron octahedra are rotated around the fourfold axis, which results in a reduction of symmetry from cubic to tetragonal.

For MgNB₉, all the B atoms in the octahedra form four bonds within the polyhedron and one bond to an icosahedron of the other layer. All bonds are typical for such a polyhedron. The B2—B3 interpolyhedron distance tends to be somewhat shorter than the intrapolyhedron bonds, which is the same as is observed in structures with the octahedral network.

The Mg coordination polyhedron consists of two spheres. The inner one is a trigonal pyramid formed by three B atoms and the N atom. These Mg—B distances are about the same as in MgB₂, namely 2.50 Å (Jones & March, 1954). The directions of the Mg—N and Mg—B bonds correlates with the smallest displacements of the Mg atom, clearly seen in the probability density function maps (Fig. 4). The Mg—B distances in the outer sphere are much longer and probably weak. Similar coordination is typical for boron-rich compounds of Mg or Al (e.g. Higashi & Ito, 1983).

S2. Experimental

Mg (99%) and amorphous B (99.99%) were used as the starting materials. A mixture of Mg and B in a molar ratio of 5:1 (total mass ~1 g) was placed in a BN crucible (inside diameter 6 mm, height 37.5 mm). The crucible was placed in a tungsten container and subjected to a high-pressure-high-temperature reaction in a high-gas-pressure apparatus (Karpinski et al., 1999). First, argon pressure was applied, and then the temperature was raised to 1873 K at a rate of 600 K h^-1. The sample was heated at this temperature for 1 h and then cooled from 1873 to 1773 K at a rate of 60 K h^-1 under 100 MPa of argon. Below 1773 K, the sample was cooled to room temperature at a rate of 600 K h^-1. After crushing the crucible, the product was heated in vacuo at 1023 K for 15 min to remove Mg. Black single crystals of MgNB₉ were extracted mechanically (maximum size 0.3 × 0.3 × 0.2 mm). Energy-dispersive X-ray diffraction analysis was performed on a Jeol 840 microscope.

S3. Refinement

The structure was refined in an isotropic approximation down to R = 0.044, then in an anisotropic approximation down to R = 0.019. Difference Fourier synthesis revealed the highest residual peaks in the vicinity of the Mg site alternating with negative regions. Anharmonic displacement parameters were refined up to the sixth order. Anharmonic atomic displacement parameters used in the program were based on the Gram-Charlier expansion of the structure factor [Cijk = Cijk × 10³, C111 = -0.0027 (3), C112 = C111/2, C113 = -0.00024 (5), C122 = –C112, C123 = C113/2, C133 = 0, C222 = –C111, C223 = C113, C233 = 0, C333 = 0.000013 (5) Units?]. All parameters of the fourth order and higher did not exceed 3σ and resulted in rather high negative regions in the probability density function maps. For this reason, only the third order anharmonic terms were refined. The probability density function map for the Mg atom is presented in Fig. 4. Such refinement reduced the R factor from 0.019 to 0.017 and the residual peaks became two times smaller Same as reduced by half?. An isotropic extinction parameter was refined. Its value was small and ~σ and it was not included in the final refinement.

Computing details

Data collection: CAD-4 Manual (Enraf-Nonius, 1988); cell refinement: CAD-4 Manual; data reduction: JANA2000 (Petřiček & Dušek, 2000); program(s) used to solve structure: CSD (Akselrud et al., 1989); program(s) used to refine structure: JANA2000; molecular graphics: ATOMS (Dowty, 1998); software used to prepare material for publication: JANA2000.

Figures

	Fig. 1. The structure of MgNB9. White circles indicate N, grey circles indicate B and dark grey circles indicate Mg.
	Fig. 2. The NB6 layer in the MgNB9 structure. White circles indicate N, grey circles indicate B and dark grey circles indicate Mg.
	Fig. 3. (a) The MgB3 layer in the MgNB9 structure and (b) the structure of CaB6 viewed along [111]. White circles indicate N, grey circles indicate B and dark grey circles indicate Mg or Ca.
	Fig. 4. The probability density function map at the Mg site, showing (a) the xy section and (b) the xz section. Contours are drawn at 1% of the highest probability and the dotted line is the 0 level.

Magnesium nitridoboride

Crystal data

MgNB9	Dx = 2.570 Mg m^−3
Mr = 135.60	Mo Kα radiation, λ = 0.71069 Å
Trigonal, R3m	Cell parameters from 24 reflections
Hall symbol: -R 3 2"	θ = 12.5–25.7°
a = 5.4960 (2) Å	µ = 0.28 mm^−1
c = 20.0873 (16) Å	T = 293 K
V = 525.47 (5) Å^3	Prism, black
Z = 6	0.22 × 0.19 × 0.10 mm
F(000) = 384

Data collection

Enraf-Nonius CAD-4 diffractometer	Rint = 0.031
θ/1.33θ scans	θmax = 50.0°, θmin = 3.0°
Absorption correction: gaussian JANA2000 (Petriček & Dušek, 2000)	h = 0→11
Tmin = 0.950, Tmax = 0.973	k = −11→10
3520 measured reflections	l = −43→43
712 independent reflections	1 standard reflections every 120 min
630 reflections with I > 3σ(I)	intensity decay: none

Refinement

Refinement on F	28 parameters
R[F^2 > 2σ(F^2)] = 0.017	Weighting scheme based on measured s.u.'s w = 1/(σ^2(F) + 0.0009F^2)
wR(F^2) = 0.035	(Δ/σ)max = 0.001
S = 1.05	Δρmax = 0.33 e Å^−3
630 reflections	Δρmin = −0.23 e Å^−3

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

	x	y	z	Uiso*/Ueq
Mg	0.0	0.0	0.30314 (3)	0.01076 (7)
N	0.0	0.0	0.19950 (2)	0.00335 (8)
B1	0.15610 (4)	0.31220 (8)	0.182668 (17)	0.00354 (8)
B2	0.22538 (4)	0.45076 (8)	0.099087 (18)	0.00364 (8)
B3	0.10607 (4)	0.21214 (8)	0.036285 (19)	0.00419 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Mg	0.01319 (10)	0.01319 (10)	0.00591 (11)	0.00659 (5)	0.0	0.0
N	0.00260 (9)	0.00260 (9)	0.00486 (15)	0.00130 (5)	0.0	0.0
B1	0.00347 (9)	0.00323 (12)	0.00384 (12)	0.00161 (6)	0.00006 (4)	0.00012 (8)
B2	0.00374 (9)	0.00357 (12)	0.00354 (12)	0.00178 (6)	−0.00015 (4)	−0.00030 (8)
B3	0.00450 (10)	0.00375 (12)	0.00406 (12)	0.00187 (6)	−0.00028 (4)	−0.00056 (8)

Geometric parameters (Å, º)

Mg—N	2.0818 (7)	N—B1	1.5240 (4)
Mg—B1	2.8398 (6)	N—B1^i	1.5240 (4)
Mg—B1^i	2.8398 (6)	N—B1^ii	1.5240 (5)
Mg—B1^ii	2.8398 (6)	B1—B1^ix	1.8055 (7)
Mg—B2^iii	2.7928 (6)	B1—B1^xi	1.8055 (5)
Mg—B2^iv	2.7928 (6)	B1—B2	1.8038 (5)
Mg—B2^v	2.7928 (6)	B1—B2^ix	1.8006 (7)
Mg—B3^iii	2.5423 (5)	B1—B2^xi	1.8006 (4)
Mg—B3^vi	2.8106 (4)	B2—B2^xii	1.7800 (4)
Mg—B3^vii	2.8106 (3)	B2—B2^xiii	1.7800 (6)
Mg—B3^iv	2.5423 (5)	B2—B3	1.6974 (5)
Mg—B3^viii	2.8106 (5)	B3—B3^i	1.7489 (4)
Mg—B3^ix	2.8106 (5)	B3—B3^xiv	1.7733 (6)
Mg—B3^v	2.5423 (5)	B3—B3^ii	1.7489 (5)
Mg—B3^x	2.8106 (2)	B3—B3^xv	1.7733 (5)
Mg—B3^xi	2.8106 (3)
N—Mg—B2^iii	158.409 (9)	B3^iv—Mg—B3^iii	94.945 (18)
N—Mg—B2^iv	158.409 (9)	B3^iv—Mg—B3^vi	131.003 (19)
N—Mg—B2^v	158.409 (10)	B3^iv—Mg—B3^viii	101.392 (16)
N—Mg—B3^iii	121.684 (14)	B3^iv—Mg—B3^ix	101.393 (16)
N—Mg—B3^vi	87.504 (14)	B3^iv—Mg—B3^v	94.94 (2)
N—Mg—B3^vii	87.504 (14)	B3^iv—Mg—B3^x	131.003 (19)
N—Mg—B3^iv	121.684 (14)	B3^viii—Mg—B3^vi	119.812 (11)
N—Mg—B3^viii	87.504 (14)	B3^viii—Mg—B3^vii	83.612 (12)
N—Mg—B3^ix	87.504 (14)	B3^viii—Mg—B3^iv	101.392 (16)
N—Mg—B3^v	121.684 (14)	B3^viii—Mg—B3^ix	155.777 (12)
N—Mg—B3^x	87.504 (14)	B3^viii—Mg—B3^v	131.003 (16)
N—Mg—B3^xi	87.504 (14)	B3^viii—Mg—B3^xi	119.812 (12)
B1—Mg—B1^i	53.893 (13)	B3^ix—Mg—B3^iii	131.003 (17)
B1—Mg—B1^ii	53.893 (15)	B3^ix—Mg—B3^vii	119.812 (11)
B1—Mg—B2^iii	170.039 (15)	B3^ix—Mg—B3^iv	101.393 (16)
B1—Mg—B2^iv	134.116 (9)	B3^ix—Mg—B3^viii	155.777 (12)
B1—Mg—B2^v	134.116 (11)	B3^ix—Mg—B3^x	119.812 (12)
B1—Mg—B3^iii	153.24 (2)	B3^ix—Mg—B3^xi	83.612 (11)
B1—Mg—B3^vi	81.691 (14)	B3^v—Mg—B3^iii	94.945 (18)
B1—Mg—B3^vii	81.691 (13)	B3^v—Mg—B3^vii	131.003 (17)
B1—Mg—B3^iv	103.000 (11)	B3^v—Mg—B3^iv	94.94 (2)
B1—Mg—B3^viii	117.36 (2)	B3^v—Mg—B3^viii	131.003 (16)
B1—Mg—B3^ix	64.755 (13)	B3^v—Mg—B3^x	101.393 (13)
B1—Mg—B3^v	103.000 (13)	B3^v—Mg—B3^xi	101.392 (12)
B1—Mg—B3^x	117.36 (2)	B3^x—Mg—B3^vi	83.612 (12)
B1—Mg—B3^xi	64.755 (11)	B3^x—Mg—B3^vii	119.812 (13)
B1^i—Mg—B1	53.893 (13)	B3^x—Mg—B3^iv	131.003 (19)
B1^i—Mg—B1^ii	53.893 (17)	B3^x—Mg—B3^ix	119.812 (12)
B1^i—Mg—B2^iii	134.116 (8)	B3^x—Mg—B3^v	101.393 (13)
B1^i—Mg—B2^iv	170.039 (15)	B3^x—Mg—B3^xi	155.777 (14)
B1^i—Mg—B2^v	134.116 (14)	B3^xi—Mg—B3^iii	131.003 (16)
B1^i—Mg—B3^iii	103.000 (12)	B3^xi—Mg—B3^vi	119.812 (11)
B1^i—Mg—B3^vi	64.755 (15)	B3^xi—Mg—B3^viii	119.812 (12)
B1^i—Mg—B3^vii	117.36 (2)	B3^xi—Mg—B3^ix	83.612 (11)
B1^i—Mg—B3^iv	153.24 (2)	B3^xi—Mg—B3^v	101.392 (12)
B1^i—Mg—B3^viii	81.691 (16)	B3^xi—Mg—B3^x	155.777 (14)
B1^i—Mg—B3^ix	81.691 (16)	B1—N—B1^i	115.22 (3)
B1^i—Mg—B3^v	103.000 (16)	B1—N—B1^ii	115.22 (2)
B1^i—Mg—B3^x	64.755 (13)	B1^i—N—B1	115.22 (3)
B1^i—Mg—B3^xi	117.36 (2)	B1^i—N—B1^ii	115.22 (3)
B1^ii—Mg—B1	53.893 (15)	B1^ii—N—B1	115.22 (2)
B1^ii—Mg—B1^i	53.893 (17)	B1^ii—N—B1^i	115.22 (3)
B1^ii—Mg—B2^iii	134.116 (12)	N—B1—B1^ix	122.316 (17)
B1^ii—Mg—B2^iv	134.116 (15)	N—B1—B1^xi	122.32 (3)
B1^ii—Mg—B2^v	170.039 (16)	N—B1—B2	124.27 (3)
B1^ii—Mg—B3^iii	103.000 (12)	N—B1—B2^ix	120.51 (3)
B1^ii—Mg—B3^vi	117.36 (2)	N—B1—B2^xi	120.51 (3)
B1^ii—Mg—B3^vii	64.755 (14)	B1^ix—B1—B1^xi	108.05 (3)
B1^ii—Mg—B3^iv	103.000 (15)	B1^ix—B1—B2	59.85 (2)
B1^ii—Mg—B3^viii	64.755 (11)	B1^ix—B1—B2^ix	60.03 (2)
B1^ii—Mg—B3^ix	117.36 (2)	B1^ix—B1—B2^xi	107.48 (3)
B1^ii—Mg—B3^v	153.24 (2)	B1^xi—B1—B1^ix	108.05 (3)
B1^ii—Mg—B3^x	81.691 (13)	B1^xi—B1—B2	59.852 (17)
B1^ii—Mg—B3^xi	81.691 (13)	B1^xi—B1—B2^ix	107.48 (2)
B2^iii—Mg—B3^vi	96.663 (16)	B1^xi—B1—B2^xi	60.028 (18)
B2^iii—Mg—B3^vii	96.663 (15)	B2—B1—B2^ix	107.30 (3)
B2^iii—Mg—B3^iv	70.621 (16)	B2—B1—B2^xi	107.297 (19)
B2^iii—Mg—B3^viii	72.011 (14)	B2^ix—B1—B2	107.30 (3)
B2^iii—Mg—B3^ix	108.320 (18)	B2^ix—B1—B2^xi	59.24 (2)
B2^iii—Mg—B3^v	70.621 (18)	B2^xi—B1—B2	107.297 (19)
B2^iii—Mg—B3^x	72.011 (12)	B2^xi—B1—B2^ix	59.24 (2)
B2^iii—Mg—B3^xi	108.320 (17)	B1—B2—B1^ix	60.12 (2)
B2^iv—Mg—B3^iii	70.621 (17)	B1—B2—B1^xi	60.120 (18)
B2^iv—Mg—B3^vi	108.32 (2)	B1—B2—B2^xii	108.46 (2)
B2^iv—Mg—B3^vii	72.011 (14)	B1—B2—B2^xiii	108.46 (3)
B2^iv—Mg—B3^viii	96.663 (17)	B1—B2—B3	116.56 (3)
B2^iv—Mg—B3^ix	96.663 (18)	B1^ix—B2—B1	60.12 (2)
B2^iv—Mg—B3^v	70.62 (2)	B1^ix—B2—B1^xi	108.48 (3)
B2^iv—Mg—B3^x	108.320 (18)	B1^ix—B2—B2^xii	108.49 (3)
B2^iv—Mg—B3^xi	72.011 (13)	B1^ix—B2—B2^xiii	60.38 (2)
B2^v—Mg—B3^iii	70.621 (16)	B1^ix—B2—B3	119.738 (17)
B2^v—Mg—B3^vi	72.011 (15)	B1^xi—B2—B1	60.120 (18)
B2^v—Mg—B3^vii	108.320 (19)	B1^xi—B2—B1^ix	108.48 (3)
B2^v—Mg—B3^iv	70.621 (19)	B1^xi—B2—B2^xii	60.379 (18)
B2^v—Mg—B3^viii	108.320 (17)	B1^xi—B2—B2^xiii	108.49 (2)
B2^v—Mg—B3^ix	72.011 (13)	B1^xi—B2—B3	119.74 (3)
B2^v—Mg—B3^x	96.663 (15)	B2^xii—B2—B2^xiii	60.00 (3)
B2^v—Mg—B3^xi	96.663 (15)	B2^xii—B2—B3	125.41 (2)
B3^iii—Mg—B3^vi	101.392 (12)	B2^xiii—B2—B2^xii	60.00 (3)
B3^iii—Mg—B3^vii	101.393 (13)	B2^xiii—B2—B3	125.41 (3)
B3^iii—Mg—B3^iv	94.945 (18)	B2—B3—B3^i	125.41 (3)
B3^iii—Mg—B3^ix	131.003 (17)	B2—B3—B3^xiv	143.267 (17)
B3^iii—Mg—B3^v	94.945 (18)	B2—B3—B3^ii	125.41 (3)
B3^iii—Mg—B3^xi	131.003 (16)	B2—B3—B3^xv	143.27 (3)
B3^vi—Mg—B3^iii	101.392 (12)	B3^i—B3—B3^xiv	90
B3^vi—Mg—B3^vii	155.777 (12)	B3^i—B3—B3^ii	60.00 (3)
B3^vi—Mg—B3^iv	131.003 (19)	B3^i—B3—B3^xv	60.454 (18)
B3^vi—Mg—B3^viii	119.812 (11)	B3^xiv—B3—B3^i	90
B3^vi—Mg—B3^x	83.612 (12)	B3^xiv—B3—B3^ii	60.45 (2)
B3^vi—Mg—B3^xi	119.812 (11)	B3^xiv—B3—B3^xv	59.09 (2)
B3^vii—Mg—B3^iii	101.393 (13)	B3^ii—B3—B3^i	60.00 (3)
B3^vii—Mg—B3^vi	155.777 (12)	B3^ii—B3—B3^xiv	60.45 (2)
B3^vii—Mg—B3^viii	83.612 (12)	B3^ii—B3—B3^xv	90
B3^vii—Mg—B3^ix	119.812 (11)	B3^xv—B3—B3^i	60.454 (18)
B3^vii—Mg—B3^v	131.003 (17)	B3^xv—B3—B3^xiv	59.09 (2)
B3^vii—Mg—B3^x	119.812 (13)	B3^xv—B3—B3^ii	90

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