Solid State Ionics 34 (1989) 93-95 North-Holland, Amsterdam SPECIFIC HEAT OF KDP NEAR THE TETRAGONAL-MONOCLINIC PHASE TRANSITION R.A. VARGAS, M. CHACON and J.C. TROCHEZ Departamento de Fisica, Universidad del Valle, Apartado Abreo 25360, Cali, Colombia Received 14 October 1988: accepted for publication 2 January 1989 Specific heat measurements of KH2PO4 (KDP) have been performed from 340 to 490 K using ac calorimetric methods. A steplike anomaly in the specific heat is observed with transition temperature 453 K on heating runs of single crystal samples, which shows the tetragonal~monoclinic phase transition of KDP. The results also show that the high-temperature monoctinic phase is metastable below 453 K. 1. Introduction 2. Experimental The phase behavior of potasium dihydrogen phos- phate (KDP) has been widely reported [ 1-4 ]. Be- low its ferro-paraelectric phase transition at 122 K (under normal pressure) is orthorhombic, while a tetragonal structure is stable at room temperature. A third crystalline modification - monoclinic - has been inferred from high-temperature X-ray studies [1], although some further refinements seem nec- essary, mainly concerning the atomic distribution within the unit cell. Other DTA, TGA 2, proton con- ductivity, dielectric constant [5] and Raman scat- tering results [4] are in favor of the existence of two high-temperature phases with different long-range crystalline order. Nevertheless, it has become ap- parent that the discrepancies in the previously re- ported results concerning the tetragonal-monoclinic phase boundary are due to inherent problems of sample preparation or its thermal history. In the present work a new attempt was made to look carefully at the thermal response of single-crys- tal and powder samples of KDP in the high-temper- ature region, seeking new information concerning the tetragonal-monoclinic phase boundary. The thermal analysis of the samples was made using the ac ca- lorimetry technique. Small single crystals of KDP were grown by the solution evaporation technique at a controlled tem- perature near 27°C and in a dry atmosphere. Merck reagent-grade KH2PO4 powder was used as starting material. Polycrystalline samples in the form of pel- lets compacted from a powder or single crystals were also used. An ac calorimeter apparatus for high temperature thermal analysis of solid samples in an inert atmo- sphere, which has been described elsewhere [ 6 ], was used to determine the temperature-dependent spe- cific heat of KDP. The samples were typically of sev- eral square millimeters in area and about 0.15 mm thickness. The temperature oscillations were in- duced in the sample by a square power input of fre- quency COo=2 Hz, produced by a beam of light chopped at this frequency. Our measured signal-to- noise ratio was best at this frequency within the cor- responding operating range which is determined by the condition that the amplitude of the temperature oscillations of the sample, I Tac I, at a fixed ambient furnace temperature, are inversely proportional to the frequency of the chopped heat pulse. The cali- bration constant of the apparatus, K, according to the basic equation of the ac calorimetry method, Cp= K/LTa,. I, was not obtained, so the reported data correspond to 1/ I ~+ I. 0 167-2738/89/$ 03.50 © Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )