R(5)Pn(3)-type phases of the heavier trivalent rare-earth-metal pnictides (Pn = Sb, Bi): new phase transitions for Er(5)Sb(3) and Tm(5)Sb(3).

The syntheses and distributions of binary R(5)Pn(3) phases among the hexagonal Mn(5)Si(3) (M), and the very similar orthorhombic beta-Yb(5)Sb(3) (Y) and Y(5)Bi(3) (YB) structure types have been studied for R = Y, Gd-Lu and Pn = Sb, Bi. Literature reports of M and YB-type structure distributions among R(5)Pn(3) phases, R = Y, Gd-Ho, are generally confirmed. The reported M-type Er(5)Sb(3) could not be reproduced. Alternate stabilization of Y-type structures by interstitials H or F has been disproved for these nominally trivalent metal pnictides. Single crystal structures are reported for (a) the low temperature YB form of Er(5)Sb(3) (Pnma, a = 7.9646(9) A, b = 9.176(1) A, c = 11.662(1) A), (b) the YB- and high temperature Y-types of Tm(5)Sb(3) (both Pnma, a = 7.9262(5), 11.6034(5) A, b = 9.1375(6), 9.1077(4) A, c = 11.6013(7), 7.9841(4) A, respectively), and (c) the YB structure of Lu(5)Sb(3), a = 7.8847(4) A, b = 9.0770(5) A, c = 11.5055(6) A. Reversible, temperature-driven phase transitions (beta-Yb(5)Sb(3) left arrow over right arrow Y(5)Bi(3) types) for the former Er(5)Sb(3) and Tm(5)Sb(3) around 1100 degrees C and the means of quenching the high temperature Y form, have been esstablished. According to their magnetic susceptibilities, YB-types of Er(5)Sb(3) and Tm(5)Sb(3) contain trivalent cations. Tight-binding linear muffin-tin-orbital method within the atomic sphere approximation (TB-LMTO-ASA) calculations for the two structures of Tm(5)Sb(3) reveal generally similar electronic structures but with subtle Tm-Tm differences supporting their relative stabilities. The ambient temperature YB-Tm(5)Sb(3) shows a deep pseudogap at E(F), approaching that of a closed shell electronic state. Short R-R bonds (3.25-3.29 A) contribute markedly to the structural stabilities of both types. The Y-type structure of Tm(5)Sb(3) shows both close structural parallels to, and bonding contrasts with, the nominally isotypic, stuffed Ca(5)Bi(3)D and its analogues. Some contradictions in the literature are discussed.

Stabilization by Hydrogen. Synthetic and Structural Studies of the Zintl Phase Ba(5)Ga(6)H(2).

Synthesis of the phase formerly reported as Ba(5)Ga(6) succeeds only in the presence of hydrogen. The heavy atom structure of Ba(5)Ga(6)H(2) has been redetermined by single-crystal X-ray diffraction (trigonal P3c1, Z = 2, a = 7.7698(2) Å, c = 14.3902(7) Å), and the hydrogen positions have been elucidated by time-of-flight neutron powder diffraction. The unit cell contains isolated slightly distorted octahedra Ga(6)(8)(-) with barium cations over all edges. Hydride is bound in two types of barium tetrahedra [d(Ba-H) = 2.61-2.62 Å]. The stoichiometry is appropriate for a Zintl phase: (Ba(2+))(5)Ga(6)(8)(-)(H(-))(2).