Trends in negative thermal expansion behavior for AMO2 (A=Cu or Ag; M=Al, Sc, In, or La) compounds with the delafossite structure

Powder neutron diffraction data were obtained from 30 to 600 K for CuAlO2, CuInO2, CuLaO2, 2H CuScO2, 3R CuScO2, and AgInO2. Rietveld refinements of these data showed negative thermal expansion (NTE) of the O–Cu–O linkage in all cases. This behavior was especially strong for CuLaO2 and CuScO2, where it persisted up to our maximum measuring temperature of 600 K. This NTE in turn caused NTE of the c cell edge, which was moderated by the positive thermal expansion of the M–O bonds. The NTE behavior increases in the CuMO2 series as the size of M increases. No NTE behavior was found for the O–Ag–O linkage in AgInO2; nonetheless, this compound did exhibit NTE for the c cell edge at low temperatures. For CuLaO2 there is NTE for both the a and c cell edges at low temperatures. Structural trends for compounds with the delafossite structure are discussed with respect to both composition and temperature.