High-Pressure Synthesis, Crystal Structure, and Magnetic and Transport Properties of a Six-Layered SrRhO3

A SrRhO3 polytype with six-layered (6M) structure was synthesized under high pressure and high temperature. The crystal structure was obtained by refining X-ray powder diffraction with the monoclinic space group C2/c with lattice parameters a = 5.5650(1) Å, b = 9.5967(2) Å, c = 14.0224(4) Å, and β = 92.846(2)°. It is isostructural with SrIrO3 synthesized under ambient pressure and consists of dimers of the face-shared Rh(2)­O6 octahedra connected by their vertices to the corner-shared Rh(1)­O6 octahedra along the c axis with a stacking of SrO3 layers in the sequence of CCHCCH, where C and H denote cubic and hexagonal closed packing, respectively. With increasing pressure, the 6M SrRhO3 transforms to an orthorhombic perovskite (Pv) phase, having a = 5.5673(1) Å, b = 5.5399(2) Å, c = 7.8550(2) Å in the space group Pbnm. A pressure–temperature phase diagram shows that the 6M-Pv phase boundary moves to lower temperatures with increasing pressure. Both the 6M and the Pv phases of SrRhO3 were characterized by magnetic susceptibility, resistivity, and thermopower; they are all metals with an enhanced and temperature-dependent magnetic susceptibility; no long-range magnetic order has been found. The polytype structures are normally found in ABO3 oxides with the geometric tolerance factor t > 1. SrRhO3 represents another example (in addition to SrIrO3) where the polytype 6M structure can be stabilized with a t < 1.