Optical Study on the Phase Transitions in KH2PO4 and KD2PO4

The infrared spectra of the PO₄ stretching vibrations have been studied on KH₂PO₄ and KD₂PO₄ over a wide range of temperature. A trend of the intensity variation with temperature for these bands is similar in each compound, but is very different between both compounds. An inspection of the intensity behaviors indicates that each PO₄ group is attached equivalently by four hydrogen atoms in KH₂PO₄, and by two deuterium atoms in KD₂PO₄, above the respective transition points. Namely, each of the hydrogen and phosphorus atoms in KH₂PO₄ moves in a single‐minimum potential well with a flattened bottom, while each of the corresponding atoms in KD₂PO₄ moves in a double‐minimum potential well. Further, non‐existence is evidenced for a Slater configuration connected with an energy parameter ${\epsilon }_0$ in his model. As a result, it is concluded that the displacive and pseudospin models are the most relevant in describing the phase transition mechanisms in KH₂PO₄ and KD₂PO₄, respectively.