Specific heat measurements in pure and in (Cu, Mn, Fe, Ni)-doped single-crystals of l-arginine phosphate monohydrate

The influence of highly diluted impurities (Cu, Mn, Fe, Ni) on the temperature (T) dependence of the specific heat (c_p) of l-arginine phosphate monohydrate (LAP) was investigated in the temperature range 1.8-300 K. The doped samples yielded values for c_p in excess to those obtained for a pure LAP sample. The melting temperatures (T_m≈420 K) obtained by differential scanning calorimetry for pure and doped LAP samples were found not to be significantly affected by the impurities. The T-dependence of c_p was fully accounted for by taking into consideration the Debye contribution, an Einstein term and a contribution due to both Frenkel and Schottky defects. The model fit all c_p versus T data using a single value for both the Debye (θ_D=160 K) and the Einstein (T_E=376.8 K) temperatures, and for the energy (ε_d=157.9 meV) required to create the defects.