Ab Initio Molecular Dynamics Simulations of (101) Surfaces of Potassium Dihydrogenphosphate

Magnetic Properties of NH4H2PO4 and KH2PO4: Emergence of Multiferroic Salts

We observe sharp step-down discontinuities in the magnetic susceptibility of NH₄H₂PO₄ and NH₄H₂PO₄-d₆₀ (60% deuterated) along the a- and c-axes occurring exactly at their antiferroelectric transition temperatures. For the case of KH₂PO₄, less pronounced discontinuities occur at the ferroelectric transition temperature. To explain this, we treat the acid protons as individual oscillators that generate current elements that translate to magnetic forces in near resonance with each other. With decreasing temperature, the resonant forces become more commensurate, which amplifies a disproportionate drop off of two types of magnetic forces to eventually trigger the structural phase transitions. For the case of NH₄H₂PO₄, the associated internal magnetic field appears to aid the NH₄⁺ to order at a higher temperature. At 49 K, a shoulder-like anomaly in both NH₄H₂PO₄ and KH₂PO₄ is attributed to a possible onset of macroscopic quantum tunneling of protons. Our findings bring forth a new category of intrinsic multiferroic systems.

Atomic dynamics of stress-induced lattice misalignment structures in a KDP subsurface

We present an ab initio molecular dynamics study of the thermal stability and dynamics behaviors of lattice misalignment structures (LMSs) in the subsurface layers of KH₂PO₄ (KDP) crystals. The dehydration process at the atomic scale is observed in the LMS system, which is the same as that in a perfect KDP crystal. However, the paths entering the dehydration process are various. The interesting result is that compared with a perfect KDP crystal, many new paths appear in the LMS system, and even in the same paths, the dehydration is more likely to happen in the LMS system. This leads to a dramatic increase in the dehydration numbers in the LMS system, for which the reasons are given in terms of structural deformation and/or uneven distribution of protons. The results elucidate the underlying atomic mechanism of the effect of LMS defects on the thermal stability of KDP material.

Compared with perfect KDP (PS), the dehydration paths and dehydration numbers in the lattice misalignment structures (LMS₁, LMS₂) increase significantly.

Atomic scale study of stress-induced misaligned subsurface layers in KDP crystals

We carried out ab initio calculations to study the atomic configuration, band structure and optical absorption of the lattice misalignment structure (LMS) in a subsurface layer of a machined KH₂PO₄ (KDP) crystal. By varying the different degrees of misalignment, the changes in the corresponding atomic position and bond and energy are obtained, and their correlations are analysed in detail. The results indicate that in the LMS evolution, the variation in the proton distribution around the oxygen atoms plays an important role, and many local stable LMSs appear. Interestingly, at a certain misalignment value, the total system energy of the local stable LMS is near that of a perfect KDP crystal. For some local stable LMSs, the electronic and optical properties related to the laser damage threshold (LDT) of KDP are further studied. The results show that in comparison with a perfect KDP crystal, the band gaps of local stable LMSs at some certain misalignment values become narrow, and their optical absorption curves produce an obvious redshift. These facts demonstrate that the emergence of the LMS could have a significant impact on the optical absorption of the KDP material and thus affect the LDT of KDP under certain working conditions.

DFT study of single water molecule adsorption on the (100) and (101) surfaces of KH2PO4

The adsorption of single water molecule on external surfaces of KDP crystal was theoretically investigated based on DFT method. We would also be grateful if you could change the image to the attached one.