Pressure-induced local lattice distortions in α-Co[N(CN)2]2

Pressure-Induced Phase Transition in Mn(Ta,Nb)2O6: An Experimental Investigation and First-Principle Study

The high-pressure and high-temperature behaviors of manganotantalite Mn(Ta,Nb)₂O₆ have been investigated by single-crystal X-ray diffraction and Raman spectroscopy combined with diamond anvil cell technique, as well as first-principle calculations. A pressure-induced reversible phase transition of manganotantalite occurs at 9.5 GPa and room temperature, accompanied by a large volume collapse (∼7.0%) and drastic color change from brownish-yellow to red. The space groups of low-pressure (LP) and high-pressure (HP) phases are the same (Pbcn), but the coordination numbers of Mn increase from six to eight and Ta increase from six to seven, respectively. The band gap becomes narrow from 2.37 to 1.59 eV. We determined the P-T phase diagram of manganotantalite with a positive Clapeyron slope of dP/dT = 0.0073 GPa/K. The P-V data were fitted to a second-order Birch-Murnaghan equation of state with B₀ = 149(4) GPa for the LP phase and B₀ = 188(3) GPa for the HP phase. The isothermal Grüneisen parameters were determined to be 0.23∼2.03 of the LP phase and 0.59∼0.86 of the HP phase for Raman modes. High-pressure behaviors of Mn(Ta,Nb)₂O₆ indicate that this kind of material is a potential effective pressure sensor to monitor pressure change or warn pressure abnormality.

Pressure-Temperature Phase Diagram Reveals Spin-Lattice Interactions in Co[N(CN)2]2

Diamond anvil cell techniques, synchrotron-based infrared and Raman spectroscopies, and lattice dynamics calculations are combined with prior magnetic property work to reveal the pressure-temperature phase diagram of Co[N(CN)₂]₂. The second-order structural boundaries converge on key areas of activity involving the spin state exposing how the pressure-induced local lattice distortions trigger the ferromagnetic → antiferromagnetic transition in this quantum material.

Local Lattice Distortions in Mn[N(CN)2]2 under Pressure

We combined synchrotron-based infrared spectroscopy, Raman scattering, and diamond anvil cell techniques with complementary lattice dynamics calculations to reveal local lattice distortions in Mn[N(CN)2]2 under compression. Strikingly, we found a series of transitions involving octahedral counter-rotations, changes in the local Mn environment, and deformations of the superexchange pathway. In addition to reinforcing magnetic property trends, these pressure-induced local lattice distortions may provide an avenue for the development of new functionalities.

Pressure-induced structural phase transformation in cobalt(II) dicyanamide

In situsynchrotron powder diffraction has been used to probe the pressure-dependent structural properties of the magnetic molecular framework material Co(dca)2[dca = dicyanamide or N(CN)2−]. An orthorhombic (Pmnn) to monoclinic (P21/n) transformation to a high-pressure phase, namely γ-Co(dca)2, occurs at 1.1 GPa. Structural determination of γ-Co(dca)2shows that the rutile-like topology of the pristine material is retained at high pressures, with the lower symmetry allowing a progression of volume-reducing structural distortions. γ-Co(dca)2was stable at the maximum pressure measured of 4.2 GPa. Both phases were soft, with bulk moduli (B0) for α-Co(dca)2and γ-Co(dca)2of 13.15 (18) and 9.0 (6) GPa, respectively. Modest uniaxial negative linear compressibility (K) of the order of −4 TPa−1was observed over the entire measured pressure range.

Non-hydrostatic behavior of KBr as a pressure medium in diamond anvil cells up to 5.63 GPa

Non-hydrostatic stresses of KBr acting as a pressure-transmitting medium have been investigated by examining their effect on a single crystal of quartz in a diamond anvil cell (DAC). The lattice strains or distortions were measured by single-crystal x-ray diffraction methods, and the non-hydrostatic deviatoric stresses for KBr were determined up to 5.63(2) GPa. The experimental results show that differences between axial stress components in the direction normal to the DAC culet face and the radial stress components in directions parallel to the DAC culet face are about 0.063(24) GPa at pressures below 2.14 GPa, and the pressure-transmitting medium can therefore be considered as quasi-hydrostatic up to this pressure. However above 2.14 GPa, after the phase transition pressure of KBr during which it converts from the B1 phase to the B2 phase, the deviatoric stresses constantly increase with increasing pressure. At the maximum pressure of this study, 5.63(2) GPa, the difference between axial stress and radial stress components reaches 0.93(9) GPa. Different variations in the non-hydrostatic deviatoric stresses were observed during both compression and decompression of the DAC, and are mainly ascribed to the phase-transition-induced volume change of KBr.