L.D. Calvert

Rare-earth bismuthides

Abstract The binary systems of the rare-earth elements (except Eu and Lu) with bismuth, have been examined using X-ray and metallographic techniques. The phases identified were RBi2 (LaBi2-type), RBi (NaCl-type), R4Bi3 (antiTh3P4-type), R5Bi3 (Mn5Si3-type), R5+xBi3 (Y5Bi3-type) and R2Bi (La2Sbtype). The LaBi2-type structure is new and single-crystal studies are reported. Refined lattice parameters are given for all the phases. Studies of the system ytterbium-bismuth are incomplete but reveal that it differs markedly from the other binary systems.

Rare-earth arsenides

Abstract X-ray powder diffraction techniques have been used to examine the binary systems formed between arsenic and the elements Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Er, and Yb, in the temperature range 500 °–900 °C. All the binary systems have a phase at the equiatomic composition which crystallises in the NaCl-type structure. In six of the systems studied (Y, Sm, Gd, Tb, Ho, Er/As) this is the only intermediate phase observed. The next most frequently occuring structure type is the NdAs2 type which is found for the diarsenides of La, Ce, Pr, and Nd. Single crystal measurements indicated that the symmetry is monoclinic, space group solP21 c . • LaAs2 (below 750 °C) a = 4.212 A , b = 6.935 A , c = 10.647 A , β = 106.60 ° • CeAs2 a = 4.165 A , b = 6.871 A , c = 10.561 A , β = 106.72 ° • PrAs2 a = 4.139 A , b = 6.844 A , c = 10.509 A , β = 106.69 ° • NdAs2 a = 4.109 A , b = 6.819 A , c = 10.449 A , β = 106.68 ° The LaAs2-type structure is formed above 750 °C only by LaAs2. Single crystal measurements indicated that the symmetry is monoclinic, space group solB2 b , a = 12.891 A , b = 9.140 A , c = 14.450 A , γ = 135.16 °. Three phases were found crystallising in the anti-Th3P4 structure, Ce4As3, Pr4As3 and Yb4As3 with a = 9.053, 8.994 and 8.791 A, respectively. A rhombohedral distortion (space group R3) of the Th3P4 structure was also observed in some samples of Yb4As3, a = 8.784 A , α = 90.80 °. The only phase obtained with the stoichiometry 5:3 was Yb5As3. It has the Mn5Si3 structure, a = 8.480 A , c = 6.671 A , solc a = 0.787 . A small range of composition extends in the direction of metal deficiency.

The rare-earth arsenides: Non-stoichiometry in the rocksalt phases

Abstract The rare-earth arsenides with the rocksalt structure are non-stoichiometric compounds. Both anion and cation lattice sites are incompletely occupied. A range of arsenic solubility is observed in all cases, and in samarium arsenide at 700 °C, it extends from Sm 0.98 As 0.81 to Sm 0.98 As 0.98 . Over part of the solubility range at the arsenic-rich end, variations in arsenic content can occur with no detectable change in lattice parameter. Further decrease in arsenic content can occur at 700 °C in all but Y, Ho, Er and Yb arsenides, and is accompanied by a reduction in the lattice parameter. A range of metal solubility exists at high temperatures for all phases and results in considerable reduction of lattice parameter at the lower metal concentrations. There is no evidence for vacancy ordering.

Rare earth arsenides: The metal-rich Europium arsenides

Abstract The binary system europium-arsenic has been investigated in the range 50 – 100 at.% europium. Numerous experimental difficulties were encountered because of the high liquidus temperatures and the extreme reactivity of the system towards container materials. Seven phases were identified. Single crystals of six phases were examined by X-ray precession methods and the powder patterns were indexed as follows: • Eu11As10 (distorted Ho11Ge10-type), orthorhombic, a = 11.255(2), b = 11.715(3), c = 17.396(4) A ; • Eu5As4, Ccmb, a = 8.0222(5), b = 15.806(1), c = 8.0586(5) A ; • Eu3As2, (Ba3P2-type, defect anti-Th3P4), I43d, a = 9.2895(4) A ; • Eu3+xAs2, I4 1 acd ,a = 16.464(5), c = 22.246(7) A ; • Eu5As3, (Ca5Pb3-type), P63mc, a = 15.245(2), c = 7.2501(7) A ; • Eu5As3, (Mn5Si3-type), P6 3 mcm , a = 8.8526(3), c = 7.0376(4) A . A ternary was also found, Eu4As2O (structure derived from La2Sb), I4 mmm , a = 4.7939(7), c = 16.1936(24). This and other new data for rare earth pnictides are reviewed. Some correlations are presented and the crystal chemistry of some of the phases is discussed.

Phase relationships and thermodynamics of refractory metal pnictides: The metal-rich tantalum arsenides

The metal-rich tantalum-arsenic system has been reinvestigated. X-ray powder diffraction techniques were used in conjunction with controlled thermal decomposition experiments in an effusion cell connected to a vacuum electrobalance. Monophase polycrystalline samples of Ta3As, Ta2As, Ta5As4 and TaAs have been prepared. Single crystals of each of these phases have been obtained by halogen vapour transport. Single-crystal precession camera studies indicated that Ta3As was of a new structure type with a monoclinic lattice and space group B2b, a = 14.683(2), b = 14.558(1), c = 5.098(1) A, γ = 90.562° (3). The structure of TaAs was refined by using single-crystal diffractometer data. The results confirm the structure assigned earlier on the basis of powder data. In addition, a modification of the TaAs phase has been identified but not, as yet, fully characterized. Powder and single-crystal data indicate a primitive tetragonal cell with structure and lattice parameters very similar to the body-centered form. n nRefined lattice parameters have been determined for all the phases. The difficulties of constructing a phase diagram for the system are discussed.

RARE-EARTH PNICTIDES: THE ARSENIC-RICH EUROPIUM ARSENIDES.

Differential thermal analysis and X-ray powder diffraction techniques have been used to examine the arsenic-rich, europium-arsenic binary system in the temperature range 600 °–1200 °C. The phases EuAs, Eu3As4, Eu2As3, EuAs2 and EuAs3 have been characterised by powder techniques and, in addition, single crystal measurements have been carried out for Eu3As4, Eu2As3 and EuAs3. EuAs, Eu3As4, Eu2As3, EuAs2 and EuAs3 decompose peritectically at 990 °, 950 °, 930 °, 912 ° and 800 °C, respectively. An additional phase Eu2As3 + x (x ⋍ 0.05) is stable between 697 ° and 842 °C. n nEuAs has a distorted NiAs structure of the Na2O2 type, P62m, a = 8.1575 A, c = 6.1378 A; the remaining phases appear to be of new structure types, Eu3As4, Fdd2, has a = 14.6438 A, b = 17.6416 A, c = 5.8857 A, Eu2As3 is monoclinic, P∗/∗, a = 14.1058 A, b = 5.9559 A, c = 12.3250 A, β = 90.74 °; EuAs2 is orthorhombic P∗∗∗, a = 7.1433 A, b = 6.9195 A, c = 6.0486 A; EuAs3 is monoclinic, C∗/∗, a = 9.500 A, b = 7.591 A, c = 5.789 A, β = 112.62 °.

The EuNi5-H system

Abstract The EuNi 5 -H system was examined using pressure-composition-temperature ( p - c - T ) methods and in situ X-ray diffractometry. One compound hydride (EuNi 5 H 5.5 ) is formed with ΔH = 26.4 kJ (mol H 2 ) −1 and ΔS = 128.5 J K −1 ( mol H 2 ) −1 obtained from the p - c - T data. The diffraction pattern can be indexed on a hexagonal lattice with a = 5.4519(21) A , c = 4.2354(24) A and V = 109.02 A 3 . The system characteristics, which include an exceptionally large value of 4.7 A 3 for ΔV per hydrogen atom, a very large pressure hysteresis and high ( x = 0.6) residual hydrogen at 298 K, are compared with those of the CaNi 5 -H and other AB 5 -H systems.

Crystal structure of CaNi5H using x-ray diffraction with in situ hydriding

Abstract The crystal structure of the lowest hydrogen content hydride in the CaNi5 + H2 system, with the approximate composition CaNi5 H, has been determined from X-ray powder diffraction data obtained using an insitu hydriding process. The metal atom positions in the body centred orthorhombic cell were determined using integrated intensity data and a block-diagonal least squares refinement.

Isotherms and crystallography of the hydrides of the system CaxEu1−xNi5

The hydriding of the system CaxEu1−xNi5 has been examined for various values of x using in-situ X-ray diffraction and pressure-composition measurement at 298 K. The single phase solutions which exist in this ternary intermetallic system for x = 0 to 0.25 and 0.8 to 1.0 exhibit hydriding behaviour closely related to that of the corresponding end member binary. Significant changes in the stability, ease of activation and the composition range of the hydrides of the ternaries compared to that of the “related” binaries are observed.