New quaternary chalcogenides balnmq₃ (ln - rare earth; m = cu, ag; q = s, se). i. structures and grinding-induced phase transition in balacuq₃

The new quaternary compounds BaLaCuQ₃ (Q = S, Se) have been synthesized by the reaction of the constituent binary chalcogenides and elements at 1000°C. The crystal structures have been determined by X-ray diffraction techniques. Crystal data: BaLaCuS₃-space group D¹⁶_2h-Pnma, M = 435.97, Z = 4, a = 11.316(2), b = 4.236(1), c = 11.724(2) Å (T = 115 K), V = 562.0(2) ų, R_w (F²) = 0.059 for 1051 observations and 38 variables, R(F) = 0.019 for 1037 observations having F²₀ > 2σ,(F²₀); BaLaCuSe₃-space group D¹⁶-Pnma, M = 576.67, Z = 4, a = 11.111(4), b = 4.293(2), c = 13.830(4) Å, (T = 115 K), V = 659.7(4) ų, R_w(F²) = 0.129 for 1385 observations and 38 variables, R(F) = 0.050 for 1313 observations having F²₀ > 2σ-(F²₀). The BaLaCuS₃ structure has a three-dimensional framework with channels that accommodate barium ions. The lanthanum atoms have mono-capped trigonal prismatic coordination and the copper atoms have tetrahedral coordination. BaLaCuSe₃ crystallizes into two phases, an α phase that is isostructural with BaLaCuS₃ and a layered β phase that is isostructural with Eu₂CuS₃. The layers, which are separated by Ba²⁺ ions, consist of edge-sharing octahedral chains and corner-sharing tetrahedral chains. The two structures are closely related and β-BaLaCuSe₃ undergoes a phase transformation to α-BaLaCuSe₃ upon mechanical grinding. The reverse phase transition occurs when α-BaLaCuSe₃ is annealed at elevated temperatures.