Precise measurements of the landau-devonshire coefficients of KDP

Abstract
The expansion coefficients α, β and γ of the free energy [image omitted] have been determined from E-P relation in a quasi-static condition. Precautions were taken to maintain a stress-free condition. The temperature control accuracy was better than ±1 mdeg. The expansion up to the sixth power describes well the first-order transition. The numerical values were [image omitted]