HEAT-CAPACITY MEASUREMENTS OF THE NORMAL-INCOMMENSURATE AND LOCK-IN TRANSITIONS IN KSC(WO4)(2) AND KSC(MOO4)(2)

The heat capacities of KSc(WO4)(2) and KSc(MoO4)(2) crystals have been measured for the first time using modulated differential scanning cal orimetry. Distinct anomalies showed up at 258.2 and 307.3 K for KSc(Mo O4)(2) and KSc(WO4)(2), respectively, revealing transition from a norm al into an incommensurate structure. The second anomaly, corresponding to an incommensurate-commensurate lock-in transition, Was found for t he KSc(WO4)(2) compound only at T-L = 290 K with a 8 K thermal hystere sis. In the case of KSc(MoO4)(2), a possible explanation for the non-o bservation of the expected anomaly due to lock-in transformation at 24 0 K might be stronger modulation wave pinning to the defects in this c rystal as compared with KSc(WO4)(2). (C) 1998 Elsevier Science Ltd. Al l rights reserved.