Temperature induced valence phase transition in intermediate-valent YbPd2Al3

Abstract

A temperature induced valence phase transition from Yb³⁺ at higher temperatures to Yb²⁺ at lower temperatures was observed at T = 110(1) K for intermetallic YbPd₂Al₃. The title compound has been prepared from the elements in sealed tantalum ampoules. The structure was refined from single-crystal data and the title compound was found to crystallize in the hexagonal YNi₂Al₃ type structure with space group P6/mmm and lattice parameters of a = 929.56(7) and c = 420.16(3) pm (300 K data). Full ordering of the Pd and Al atoms within the [Pd₂Al₃]^δ− polyanion was observed. Magnetic measurements revealed an anomaly in the dc susceptibility data and intermediate valent Yb at higher temperature, as observed from the effective magnetic moment. The proposed valence phase transition was also observed as a λ-type anomaly in heat capacity measurements (T = 108.4(1) K), however, no systematic shift of the λ-peak was observed in field dependent heat capacity measurements. An antiferromagnetic ordering at this temperature, however, could be excluded, based on field-dependent susceptibility measurements and magnetization isotherms. No dynamic phenomenon was observed in ac susceptibility measurements, excluding e.g. spin-glass behavior. Subsequent temperature dependent single-crystal and powder X-ray diffraction experiments indicated a steep increase in the length of the c axis around T = 110 K upon cooling. However, no structural phase transition was found via single-crystal diffraction experiments conducted at 90 K. The anomaly was also observed in other physical measurements of e.g. the electrical resistivity, indicating a clear change in the electronic structure of the material. X-ray photoelectron spectroscopy conducted at room temperature shows the presence of both, Yb²⁺ and Yb³⁺, underlining the mixed-valent state. Members of the solid solution Yb_1−xCa_xPd₂Al₃ (x = 0.33, 0.67, 1) were finally used to further study the charge ordering and the present temperature induced valence phase transition.