Critical behavior of crystal lattice of NaV2O5

Abstract Critical behavior of crystal lattice was investigated by X-ray scattering for both stoichiometric NaV 2 O 5 and Na-deficient Na x V 2 O 5 ( x =0.99–0.96). Critical exponents were determined from superlattice reflections with a wave number vector Q =(1/2,1/2,1/4), specifying a lattice modulation formed below T C =35 K. The previously observed anomalous lattice strain below T C was found to be a secondary order parameter driven by a primary order parameter which is an atomic displacement with Q governing the low-temperature crystal structure. The temperature–concentration phase diagram was also determined and the transition temperature was found to decrease linearly with increasing deficiency x as d T C / d x=4.2 ( K /%).