Advanced Centre of Research in High Energy Materials (ACRHEM), University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, Hyderabad-500046, Telangana, India.
Phys Chem Chem Phys. 2019 Jan 2;21(2):884-900. doi: 10.1039/c8cp04827d.
The potential crystal structures and properties of urea as a function of pressure were studied using ab initio based electronic structure calculations. The enthalpy-pressure behavior shows that urea undergoes a pressure induced structural phase transition from P4[combining macron]21m (phase I) → P212121 (phase III) at 0.66 GPa with a volume collapse of 4.83%, driven by softening of the acoustic mode along the Γ-X direction. Another phase transition from the P212121 → P21212 structure was identified at 3.09 GPa. The violation of Born stability criteria in the P212121 structure along with softening of the acoustic mode in the U-R direction was responsible for the pressure induced phase transition. Furthermore, the application of pressure led to the breaking and formation of N-HO bonds in the crystal structure of urea during the phase transition, i.e., the H-acceptor capacitance of the oxygen atoms was varied between phases I/IV and -III. Band structure calculations were performed using a hybrid functional (Heyd, Scuseria and Ernzerhof, HSE) which includes a part of exact Fock-exchange. The computed electronic band structure showed that the urea polymorphs are insulators with a direct band gap of 6.21, 6.85 and 6.99 eV for phase-I, -III and -IV, respectively, at selected pressures. We have also presented the dielectric functions (real (ε1(ω)) and imaginary (ε2(ω)) parts), refractive index and absorption coefficients to explore the optical characteristics of the urea phases. The geometric interpretation of intermolecular interactions were quantitatively visualized using Hirshfeld surface analysis. Our results provide a complete picture of various properties of urea polymorphs that lay the foundation for further understanding of structures and their applications.
采用基于第一性原理的电子结构计算研究了尿素在压力下的潜在晶体结构和性质。焓-压行为表明，尿素在 0.66GPa 时经历了由 P4[combining macron]21m（相 I）→P212121（相 III）的压力诱导结构相变，体积收缩 4.83%，这是由 Γ-X 方向的声学模软化驱动的。在 3.09GPa 时还确定了另一个从 P212121→P21212 结构的相变。在 P212121 结构中违反 Born 稳定性准则以及在 U-R 方向上声学模软化是导致压力诱导相变的原因。此外，在相变过程中，压力的施加导致尿素晶体结构中 N-HO 键的断裂和形成，即在相 I/IV 和 -III 之间，氧原子的 H-受体电容发生变化。使用包含部分精确 Fock 交换的杂化泛函（Heyd、Scuseria 和 Ernzerhof，HSE）进行了能带结构计算。计算的电子能带结构表明，尿素多晶型物是绝缘体，在选定压力下，相 I、-III 和 -IV 的直接带隙分别为 6.21、6.85 和 6.99eV。我们还提出了介电函数（实部（ε1(ω)）和虚部（ε2(ω)））、折射率和吸收系数，以探索尿素相的光学特性。使用 Hirshfeld 表面分析对分子间相互作用进行了定量可视化。我们的结果提供了尿素多晶型物的各种性质的完整描述，为进一步理解结构及其应用奠定了基础。
