IIb: Prl, Pr2, Pr3, Pr4, Pr6 (2x)) share two rectangular faces and one edge with the surrounding prisms, as can be seen from Fig. 2. The Pr prisms of type I and IIa ...
Solid
State Sciences
t. 1,
1999, p. 509-521
PrsC17B7 : preparation, H. MATTAUSCH,
structure, bonding, properties 0. OECKLER,
G. V. VAJENINE,
R. K. KREMER and A. SIMON Max-Planck-Institut fiir FestkGrperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany
(received May 31, 1999; accepted June 7, 1999.)
Dedicated to Dr Marcel SERGENT on the occasion of his retirement
ABSTRACT.PraC17Br is prepared from stoichiometric mixtures of PrC13, Pr and B at 1220 K in closed Ta capsules. PraCl~Br forms golden coloured needles sensitive to moist air. It crystallizes in the space group Pi with a = 773.1(2) pm, b = 903.0(2) pm, c = 1419.4(3) pm, CL= 81.55(3)0, B = 82.18(3)‘, and y = 64.76(3)0. ln the crystal structure Prb trigonal prisms are condensed to double chains which run parallel [loo]. Some of the prisms and rectangular prism faces are centered by boron atoms which leads to By, Bg, Bs rings, and Bz dumbbells condensed into ribbons. These PrsBT strands are surrounded and held together by the Cl atoms. PrsClrB, is a metallic conductor and shows Curie Weiss behavior with bff = 3.48 pa. According to extended Hiickel calculations, the distribution of valence electrons is best described by a formulation Prs’a+C177B711-. Bonding within the boron ribbons is thus nearly optimal, while the average 4fz 5d3’4 configuration of Pr accounts for both the observed magnetic moment and metallic conductivity.
INTRODUCTION In the rare earth halide box-ides REX,,B, [I] the dimensionality of the extended boron substructures is frequently reduced as compared to the binary rare earth borides REB, [2, 31, and one-dimensional, quasimolecular or atomic boron entities result. In the case of a sufficient number of metal valence electrons discrete B atoms center RE6 octahedra. Gd$&B [4], Solid State Sciences, 1293-2558/99/7-S/0
1999 fiditions scientifiques et mddicales Elsevier SAS. All rights reserved.
510
H. MATTAUSCH
et al.
Gd&B, Y4Brd3 [s]; and Sc$&B [6] represent examples with three- and one-dimensionally condensed as well as discrete RE,j units. The crystal structures of these RE halide borides are isotypic with the corresponding halide carbides. In contrast, extended B-B bonding is observed in the compounds REaSB4 [7]. The characteristic motif of these structures are B4 rhomboids, which are connected to chains via direct B-B bonds. Here we report the new compound PrsC17B7. The boron atoms form B3, Bg, and Bs rings as well as B2 dumbbells. These entities are connected in one dimension to form B strands. PrsC17B7 shows metallic conductivity in accordance with band structure calculations. EXPERIMENTAL
DETAILS
A - Preparation Pr metal (sublimed, 99.99 %; Alfa / J. Matthey) was crushed into small lumps under Ar atmosphere in a drybox. PrC& was prepared from PrbOrr (99.9 %, Alfa / J. Matthey) which was dissolved in HCl, reacted with NH&l [8, 91 and distilled under vacuum in tantalum containers. Boron (99.4 %, Ventron) was heated in vacuum at 1400 K for 12 h prior to use. Stoichiometric mixtures of PrQ, Pr, and B (molar ratio 7 : 17 : 21) were reacted in sealed Ta tubes under Ar at 1220 K for 18 d with quantitative yield according to X-ray powder diagrams. PrsC1,B7 forms golden coloured needles sensitive to moist air. All measurements had to be done under Ar or He atmosphere. B - X-ray Structure Investigation PrsC17B7 was characterized by X-ray powder diffraction (modified Guinier technique [lo]; k(CuKor) = 154.056 pm, internal standard Si, a = 543.035 pm, image plate (Fuji BAS5000)). Crystals suitable for single-crystal investigation were sealed in glass capillaries and selected by Weissenberg and Buerger photographs. Details on the data collection and structure refinement are summarized in Table I. The positions of the Pr and Cl atoms were derived by direct methods, the B atoms were localized in difference Fourier maps. The refined atomic parameters are shown in Table II, the anisotropic displacement parameters in Table III, and the shortest interatomic distances together with the angles in the boron rings in Table IV.’
I Further details on the crystal structure may Karlsruhe, D-76334 Eggenstein-Leopoldshafen names of the authors, and the journal citation. TOME
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be obtained from the Fachinfonnationszenb upon quoting the number CSD-410728,
the
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:PREPARATION,STRUCTURE,BONDING,PROPERTIES
C - Electrical Conductivity The electrical conductivity between 4 K and room temperature was measured according to the van der Pauw method [I 11 on disc shaped pellets (8 mm diameter, 1 mm thickness) which had been sintered at 1220 K in Ta tubes under Ar atmosphere. D - Magnetic Susceptibility The magnetic susceptibility was determined with a MPMS magnetometer (Quantum Design) in a field of 1 T on a powder sample (approx. 150 mg) between 2 K and room temperature. E - Band Structure Calculation Electronic structure calculations were carried out using the extended Hiickel molecular orbital method [IZ, 13, 14, 151 with the following atomic parameters for B [16], Cl [l7], and Pr [18] (orbital energies in eV with orbital exponents for the Slater-type functions given in parentheses): B 2s -15.2 (1.3), B 2p 8.5 (1.3), Cl 3s -26.3 (2.183), Cl 3p -14.2 (1.733), Pr 6s -7.42 (l-40), and Pr 6p -4.65 (1.40). The Pr 5d wavefunctions (at -8.08 eV) were represented by a sum of two Slater-type functions with exponents of 2.75 and 1.267 and weights of 0.7187 and 0.4449. Pr 4f wavefunctions were not explicitly included in the calculations, but the total number of valence electrons was reduced from five to three, effectively localizing two electrons in 4f states which do not participate in bonding. A set of 40 k-points was used to calculate the density of states (DOS) and crystal orbital overlap population (COOP) for the model one-dimensional Pr&l& chain. Table I Crystal data and structure refinement of PrsC17Br [ 19,201 Formula weight Colour, Habitus
1451.10g/m01 Golden needle
Crystal dimension Temperature
0.05 x 0.05 x 0.3 mm3 293(2) K
Radiation Crystal system Spacegroup Cell dimensions
MoKa, 71.073 pm Triclinic
Volume I Z SOLID STATESCIENCES
pi
a= 773.1(2)pm b = 903.0(2) Pm C = 1419.4(3) pm 0.8836(3)nm3!2
o=81.55(3)” p = 82.18(3) 7 = 64.76(3)
512
H. MATTAUSCH
Density (calculated) Absorption coefficient Diffractometer (pstart,
vend,
cpiicr.
8 range index ranges Measured reflections Independent reflections Absorption correction Structure refinement Data / restraints / parameters Goodness of fit on F2 R values [I > 20 (I)] R values (all data) Final Fourier residuals
f?t al.
5.454 g/cm3 22.633 mm-’ Stoe IPDS O.O”, 190.8”, 1.8” 2.91” to 30.37” -91hI IO,-12lkl12,-1951520 9809 4828 (Ri,t = 0.0408) Semiempirical Full matrix least-squares on F* 4828 J 0 J 168 0.861 Rl = 0.03 17, wR2 = 0.0444 RI = 0.0754, wR2 = 0.0517 1582 and-1559 enmM3
Table II Atomic coordinates (x 104) and equivalent isotropic displacement parameters (pm*) of PrsC1,B7. U(eq) is defined as one-third of the trace of the orthogonalized IJu tensor.
Prl Pr2 Pr3 Pr4 Pr5 Pr6 Pr7 Pr8 Cl1 Cl2 Cl3 Cl4 Cl5 Cl6 Cl7 Cl8 Bl B2 B3 B4 TOME
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3610(l) -1190(l) 1357(l) -3453( 1) 1179(l) 6117(l) 102(l) -4924( 1) 2647(4) -2416(4) 0 5000 6055(4) -1102(4) -3275(4) 1786(4) -4928(19) 79(18) -2094(17) 5936( 14)
1579(l) 1581(l) 5804( 1) 5804( 1) 2809( 1) 2920( 1) 2661(l) 2747(l) -1347(4) -1214(4) 0 0 -169(4) 161(4) 361 l(4) 3654(4) 3668(15) 3553( 14) 5389(13) 5790( 11)
2002( 1) 1999(l) 2577( 1) 2576( 1) -333( 1) -312(l) 4268( 1) 4203( 1) 2089(2) 2114(2) 0 0 6022(2) 3977(2) 5712(2) 5696(2) 2360(9) 2527(8) -684(7) -1256(7)
7-8
UG-% 94(l) 95(I) 101(l) 98(l) 76(l) 800) 105(l) 103(l) 119(6) 132(6) 133(8) 121(8) 140(6) 133(6) 146(6) 140(6) 1lO(20) 90(20) 130(20) 80(20)
PrsC17B7 BS B6 B7
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:PREPARATION,STRUCTURE,BONDING,PROPERTIES
-3885(E) 2117(17) -63(17)
4210(12) 5365( 13) 5803(13)
f268(7) -676(7) -1333(7)
1OO(20) 120(20) 140(20)
Table III Anisotropic atomic displacement parameters Uii (pm*) for Pr&17B7. The anisotro& displacement factor is defined as: exp(-2x2 [(ha*)*U,, + . .. + 2hka*b*U12]} UII 122(3) 117(3) 126(3) 116(3) 80(3) 95(3) ill(3) 107(3) 118(14) 152(15) 140(20) lOO(20) 152(15) 167(15) 186(15) 134( 14)
prl Pr2 Pr3 Pr4 Pr5 Pr6 Pr7 Pr8 Cl1 Cl2 Cl3 Cl4 Cl5 Cl6 Cl7 Cl8
SOLID
STATE
SCIENCES
u22
W9 WY 6X3) 69(3) 64(3) 7W) 114(3) 113(3) llO(13) 129(13) PO(20) 120(20) 145(15) 105(14) 125( 14) 144( 14)
u33
W3) 89(3) 95(3) lOl(3) 74(3) 71(3) 91(3) 89(3) 127(12) 116(12) 170(20) 120(20) 134(14) 1 lO(13) 120(13) 136(14)
u23
4x2) O(2) -19(2) -1 l(2) -3(2) -5~2) -26(2) -27(2) -23(P) -23(10) lO(20) 7(15) -11(11) 15(10) -23(11) -311)
u13
-16(2) -18(2) -1 l(2) -17(2) -17(2) -14(2) a(3) -w -7(10) -12(10) -50(20) -25(15) -12(11) -2O(ll) -24( 12) -21(11) \ ,
u2
-23(2) -22(2) -19(2) -27(2) -18(2) -32(2) -44(3) -41(3) -43(10) -56(11) -50(20) -30(20) -72( 12) -45( 11) -52( 12) -53( 11)
514
H. MATTAUSCH
RESULTS
et cd.
AND DISCUSSION
CRYSTAL STRLJCTIJRE.- The crystal structure of PrsC17B7 is shown in a projection along w [loo] in Fig. 1. The Pr atoms form trigonal prisms, which are connected through two rectangular prism faces to form linear chains. Two such single chains are condensed via rectangular faces to double chains. Additional Pr atoms (Pr?, Pr8) cap further rectangular faces on both sides of the strands in tetragonal pyramidal fashion. In Fig. 1 we look along these ribbons. The arrangement of the Pr atoms of one strand is drawn in a perspective view on the lower right hand side of Fig. 1.
Fig. 1 - Projection of the crystal structure of PrSC17B7along = [IO@& The Cl, Pr and 3 atoms are drawn as spheres with decreasing size. The unit cetl is outlined. One strand consisting of a double chain of prisms and tetragonal pyramids of Pr atoms is shown in a perspective view on the lower right hand side.
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Pr, prisms of one kind (Ia: Prl, Pr3, Pr5 (2x), Pr6 (2x), Ib: Pr2, Pr4, Pr5 (2x), Pr6(2x)) are fused with three neighbouring prisms through all rectangular faces, prisms of the other kind (IIa: Prl, Pr2, Pr3, Pr4, Pr5 (2x); IIb: Prl, Pr2, Pr3, Pr4, Pr6 (2x)) share two rectangular faces and one edge with the surrounding prisms, as can be seen from Fig. 2. The Pr prisms of type I and IIa are centered by B atoms, and B6 rings result (compare Fig. 2) as in the REB2 structures [21]; however, they are not extended twodimensionally. The Bs rings have additional boron atoms (B2) on both sides in para position forming Bz dumbbells. These B atoms (B2) occupy half of the Pr pyramids. Such environment is very common for the B-C group in rare earth carbide borides [22, 231 and rare earth halide boride carbides [24] with the B atom in trigonal prismatic and the C atom in pyramidal surroundings by RE atoms. The presence of C instead of B can be excluded, as PrsC17B7 can be prepared in quantitative yield without C. For type III> prisms all rectangular faces are centered by B atoms, and Bs rings result (see Fig. 2). This rectangular planar coordination of boron by the RE atoms is quite unusual. However, in the course of our investigations we observed a very similar coordination of B by the RE atoms in other halide borides REX,,B, like Gd2C12B3 [z]. As shown in Fig. 2, linking of the B3 and B6 rings results in eight membered boron rings. Chains of alternating B6 and Bs rings are running parallel [ 1001 through the crystal.
Fig. 2 - Perspective view on a characteristic section of a strand parallel [ 1001 consisting of chains of B6 and Be rings, B3 and B2 units (B: small, gray spheres), embedded in double chains of trigonal prisms of Pr atoms with the additional capping Pr atoms (large, black spheres). The Cl atoms are omitted. SOLID
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f?t d.
Table IV Selected interatomic Prl -Pr2 Prl - Pr3 Prl -Pr5 Prl - Pr7 Prl -Pr8 Prl -Cl1 Prl -Cl2 Prl -Cl4 Pr 1 - Cl5 Prl -Bl Prl - B2 Prl -B3 Prl - B4 Prl -B7 Pr2 - Pr4 Pr2-Pr5 Pr2 - Pr6 Pr2 -Pr7 Pr2 -Pr8 Pr2 - Cl1 Pr2 - Cl2 Pr2 - Cl3 Pr2 - Cl6 Pr2-Bl Pr2 - B2 Pr2-B5 Pr2-B6 Pr2 - B7 Pr3 - Pr4 Pr3 - Pr5 Pr3 -Pr6 Pr3 - Pr7 Pr3 - Pr8 Pr3 - Cl 1
371.1(2) 362.2(2) 381.9(2) 386.6(2) 388.65(13) 301.7(3) 303.8(3) 321.18(12) 290.2(3) 271.1(13) 262.2( 12) 295.4(10) 260.0( 10) 292.1(11) 362.2(2) 381.4(2) 388.6(2) 386.7(2) 389.2(2) 302.0(3) 302.8(3) 320.79( 12) 290.8(3) 272.1(13) 260.4( 12) 258.9(10) 296.7( 10) 290.9( 12) 37 1.9(2) 374.0(2) 382.1(2) 380.4(Z) 379.2(2) 309.4(3)
B5 -Bl -B4 B4-B3-B7 B4-B3-B6 B7-B3-B6 B5-B4-B3 B5 - B4 - Bl TOME
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Pr3 -Cl2 Pr3 - Cl7 Pr3-Cl8 Pr3-Bl Pr3-B2 Pr3-B3 Pr3-B4 Pr3 - B7 Pr4 - Pr5 Pr4 - Pr6 Pr4 - Pr7 Pr4 - Pr8 Pr4-Cl1 Pr4 - Cl2 Pr4-Cl7 PI-~-Cl8 Pr4-Bl Pr4-B2 Pr4 - B5 Pr4-B6 Pr4 - B7 Pr5 - Pr5 Pr5 - Pr6 Pr5 - Cl2 Pr5 - Cl3 Pr5 - Cl4 Pr5 - B3 Pr5 - B3 Pr5 - B6 Pr5 - B6 Pr5 - B7 Pr5 - B7 Pr6 - Pr6 Pr6-Cl1
308.1(3) 322.4(3) 314.1(4) 269.7(13) 262.5( 12) 293.6(10) 266.9( 10) 295.0( 11) 373.9(2) 381.9(2) 381.0(2) 378.6(2) 310.1(3) 308.3(3) 318.5(4) 310.8(3) 269.5( 13) 261.7(12) 264.5(9) 293.3(10) 294.9(12) 378.9(2) 386.6(2) 293.5(3) 299.85( 10) 300.3(2) 265.6( 11) 268.0(11) 266.3(11) 271.7(11) 269.7(10) 270.2( 11) 359.3(2) 292.1(3)
B3-B4-Bl B4-B5-B6 B4-B5-Bl B6-B5-Bl B5-B6-B7 B5-B6-B3
145.7(9) 147.9(8) 63.2(7) 148.5(8) 118.3(7) 122.7(9)
Pr6-Cl3 Pr6 - Cl4 Pr6 - B3 Pr6 - B3 Pr6-B4 Pr6-B4 Pr6-B5 Pr6-B5 Pr6 - B6 Pr6 - B6 Pr7 - Pr8 Pr7 - Pr8 Pr7 - Cl5 Pr7 - Cl6 Pr7 - Cl6 Pr7 - Cl7 Pr7 - Cl8 Pr7 - Cl8 Pr7 - B2 Pr8 - Cl5 Pr8 - Cl6 Pr8 - Cl7 Pr8 - Cl7 Pr8 - Cl8 Pr8-BI Bl -B4 Bl -B5 B2-B7 B3-B4 B3-B6 B3-B7 B4-B5 B5-B6 B6-B7
307.1(2) 306.46( 10) 304.1(12) 306.5( 12) 265.6(10) 269.3( 10) 267.5(9) 270.6(9) 299.7(11) 303.2( 11) 386.68( 14) 386.73(14) 288.0(3) 288.2(3) 320.6(3) 298.4(4) 297.6(3) 301.5(3) 248.3( 11) 288.3(3) 289.2(3) 298.5(3) 299.0(3) 298.3(4) 262.6( 12) 176(2) 175(2) 171(2) 170(2) 195.1(13) 186(2) 158.8(14) 166(2) 190(2)
B7-B6-B3 B2 - B7 - B3 B2-B7-B6 B3-B7-B6
118.8(9) 120.5(8) 119.9(9) 119.4(7)
PrsC17B7 :PREPARATION,STRUCTURE,BONDING,PROPERTIES
517
In contrast to the relatively regular B6 rings with distances of 186 pm < du+ I 195 pm and angles within the rings of 119” < La.+a I 122”, the Bs rings are elongated in the chain direction. The angles and distances vary between 118” and 15 lo, and between 159 pm and 195 pm, respectively. Formally, it is possible to construct comparable Bs rings by cutting the crystal structure of REB6 [26] parallel (001). For PrB6 [27] the B-B distances in the Bs units are in a similar range (165 pm I d&a < 175 pm) as in PrsC17B7. Short B-B separations are found in the boron triangle with 159 pm (and 175 pm - 176 pm for the other two edges), and in the B2 unit with dsB = 171 pm. Analyzing the B-B distances in terms of Pauling bond orders with d, = dr 60 log n and d, = 159 pm [28] one calculates 0.3 < n < 1.O for dga = 195 pm - 159 pm. Hence, the B atoms B4 and B5 of the boron triangle are left with a ‘valency‘ of 0.7 and 0.8, respectively, for Pr-B bonding, and they are the only ones with the coordination number 4 with respect to Pr. The atom B2 of the dumbbell is left with the high value of 2.3, higher than for the comparable atom Bl of the triangle (1.9), which is reflected in the short averaged distance daZ-pr = 259.0 pm as compared to dB1+ = 267.4 pm. On the other hand, atom B2 is moved toward the center of gravity of the Prs pyramid resulting in the shortest Pr-B distance in the structure of dB2-prT= 248.3 pm. The corresponding distance for Bl is da,+, = 262.6 pm. The remaining ‘valences‘ of the B atoms of the B6 rings to Pr are all = 1.7. The Cl atoms surround the Pr-B-Pr strands taking positions above the triangular faces of the Pr prisms and pyramids and above the corners of the pyramids marked as Cl’ and Cl”, respectively, following Schafer’s notation [29]. In the b-c plane. of the crystal structure (see Fig. 1) the strands are interconnected by Cl”‘, Cl’” and Cl”’ contacts. The PrCl bonding distances are in the expected range of 288 pm I dprcl I 321 pm. PHYSICAL PROPERTIES.Above 50 K the magnetic susceptibility follows a Curie Weiss law as indicated by the plot of the reciprocal molar susceptibility with a paramagnetic Curie temperature 0 = -22(l) K indicating predominant antiferromagnetic exchange interaction. The Curie Weiss parameter C corresponds to an effective magnetic moment of 3.48(5) pa in a very good agreement with the expected value of 3.58 pa (4f2 configuration) for Pr in an oxidation state of +3. The susceptibility exhibits a local maximum at 33(l) K indicating the Neel temperature TN below which antiferromagnetic ordering appears. The electrical resistivity proves metallic behavior with decreasing resistivity from room temperature to 40 K. The room temperature resistivity amounts to approximately 1 r& cm. A local minimum in the resistivity appears close to the magnetic ordering temperature, and an increase of approximately 20% is observed below. SOLJD
STATESCIENCES
518
H. MATTAUSCH
100
"0
et d.
200
300
T(K) T(K) 1.15 1.10 1.10
g1.05r
/ 8E
..
;
1.00.
E.
0.95
-
.bf
l
CL
8 0.90
-
0.85 0
l
l*
0.
‘s”’ 100
200
300
T (4 Fig. 3 - Inverse molar magnetic susceptibility (top) and electrical resistivity (bottom) of Pr&B,.
BAND STRUCTURE CALCULATION.The Zintl Klemm concept is a useful starting point for the analysis of chemical bonding in Pr8C17B7. The above-mentioned magnetic data suggest the 3+ assignment for Pr, leading to a formulation Pr,24+C1~-B7’7W. The formal charge on the boron sublattice can be precisely accounted for if the n: system of the boron ribbons is completely filled corresponding to B-B single bonds: [Prs]24+[C1,]7[‘*yf 1]3-[“b’B2]~[‘3b’B3]*-[(3b)B4]*-[(3b)B5]*-[(3b)B6]*~(3b)B7]*- _ p#+&-
Semiconducting behaviour should result; however, metallic . behaviour is observed. This apparent contradiction can be resolved if electron transfer from the anionic sublattice to the RE atoms is taken into account. For example, partial electron transfer from the x* states of the C2
B7
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unit to the empty Gd 5d orbitals in Gd2Br2C2, which, according to the Gd3+2Br-&!24^ formalism, should be a semiconductor, is responsible for metallic conductivity [30]. According to our extended Htickel calculations the electron transfer in PrsC17B7 is even more pronounced. Fig. 4 presents the DOS and COOP plots for a one-dimensional PrsCli4B7 chain, carved out from the full threedimensional PrsCl~B7 structure as shown in Fig. 1 (lower right comer). In order to ensure the same electron count for the PrsB7 strand, a charge of 7has to be added along with seven Cl ligands. The Fermi level for the resulting PrsCl&r’is labeled by a in Fig. 4. The states immediately below the Fermi level are mostly Pr in character, not B, as one would expect based on the above-mentioned formalism. In fact, six electrons per Pr8C17B7 formula unit are transfered (if the Prs24’C177-B717- assignment is used as a starting point) from the rc* states of the boron ribbon to the Pr 5d states, resulting in the formulation Prg18+C17713711-. The Fermi level for the Pr8C114B71- (marked by b in Fig. 4), derived from the hypothetical Pr8C17B7@, corresponds to complete filling of B-B bonding states with B-B antibonding states (both cr* and rt*) remaining mostly unfilled. Thus the B-B bonding is close to optimal. Pr
6
B-0 COOP
Fig. 4 - The total DOS and contributions of Pr (left panel) and B (center panel) atoms to the DOS of the one-dimensional Pr&l14B7 chain (the Pr and B contributions are lined). The average B-B COOP is shown in the right panel. The horizontal dashed lines indicate the positions of the Fermi level for PrsCl14B7’- (a), representative of Pr&l,B,, and PGJCI~~B~~-(b), representative of PE$J~B~~. The completely tilled Cl 3s and 3p states lie below -12 eV.
As a result of the electron transfer, the average Pr configuration written as 4f 25d3’4, with the localized 4f electrons contributing SOLID STATE SCIENCES
can be to the
520
H.MATTAUSCH
magnetic moment of Fr and delocalized metallic conductivity.
et&
5d electrons being responsible for
ACKNOWLEDGEMENT The authors wish to thank E. Bticher, for experimental help.
R. Eger, C. Kamella,
and G. Siegle
REFERENCES [l] [2] [3] [4]
HJ. MATTAUSCH, 0. OECKLER, A. SIMON, Znorg. Chim. Acta, 1999,287,173. GmeEin Handbook RE Main; Springer Verlag: Berlin, 1990, Vol. C Ila. J. ETOURNEAU, .J. Less-Common Met., 1985,110,267. A. SIMON, HJ. MATTAUSCH, G. J. MILLER, W. BAUHOFER, R. K. KREMER, Handbook on the Physics and Chemistry of Rare Earths, K. A. Gschneidner Jr., L. Eyring, Eds.; Elsevier Science Publishers: New York, 1991, Yol. 15, p. 191. [5] HJ. MATTAUSCH, A. SIMON, Z. Kristallogr., 1997,212,99. [6] D. S. DUDIS, J. D. CORBETT, Inorg. Chem., 1986,25,3434. [7] HJ. MATTAUSCH, A. SIMON, C. FELSER, J. Phys. Chem. B, 1997, 101,995l. [S] A. BRUKL,Angew. Chem., 1939,52,152. [9] G. MEYER, P. AX, Muter. Res. Bull, 1982, 17, 1447. [lo] A. SIMON, J. A&. CqvtaZlogr., 1970,3, 11. [ 1 l] L. J. VAN DER PAUW, Philips Res. Rep., 1958,13, 1. [ 123 G. A. LANDRUM, Yet Another Extended Hiickel Molecular Orbital Package (YAeHMOP), Cornell University: 1997. YAeHMOP is freely available on the World Wide Web at http:Noverlap.chem.comell.edu:8O8O/yaehmop.htrnl. [13] R. HOFFMANN,J. Chem. Phys., 1963,39, 1397. [ 141 R. HOFFMANN, Solids and Surfaces: A Chemist’s View of Bonding in Extended Structures, VCH Publishers: New York, 1988. [ 151 J. K. BURDETT, Chemical Bonding in Solids, Oxford University Press: New York, 1995. [ 161 A. B. ANDERSON, R. HOFFMANN, J. Chem. Phys., 1974,60,427 1. [ 171 R. H. SUMMERVILLE, R. HOFFMANN, J. Am. Chem. Sot., 1976,98,7240. 1181 Y. TIAN, T. HUGHBANKS, Znorg. Chem., 1993,32,400. [ 191 G. M. SHELDRICK, SHELXS97, Program for the Solution of Crystal Structures; Universitlt Giittingen: 1997. [203 G. M. SHELDRICK, SHELxL97, Program for the Refinement of Crystal Structures; Universitgt GBttingen: 1997. [21] K. H. J. BUSCHOW, Boron and Refractory Borides, V. I. Matkovich, Springer Verlag: Berlin, 1977,494. [22] F. WIITKAR, S. KAHLAL, J.-F. HALET, J.-Y. SAILLARD, J. BAUER, P. ROGL, Inorg. Chem. 1995,34, 1248. [23] J. BAUER, H. NOWOTNY,Monatsh. Chem., 1971,102, 1129. [24] HJ. MATTAUSCH, A. SIMON, Angew. Chem., 1995,107,1764; Angew. Chem. Int. Ed. Engl., 1995,34, 1633.
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:PREPARATION,STRUCTURE,BONDING,PROPERTIES
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