The origin of the anomalous, 400% increase of the piezoelectric coefficient in Sc(x)Al(1-x)N alloys is revealed. Quantum mechanical calculations show that the effect is intrinsic. It comes from a strong change in the response of the internal atomic coordinates to strain and pronounced softening of C33 elastic constant. The underlying mechanism is the flattening of the energy landscape due to a competition between the parent wurtzite and the so far experimentally unknown hexagonal phases of the alloy. Our observation provides a route for the design of materials with high piezoelectric response.

Poly-nitrogen compounds have been considered as potential high energy density materials for a long time due to the large number of energetic N–N or N=N bonds. In most cases high nitrogen content and stability at ambient conditions are mutually exclusive, thereby making the synthesis of such materials challenging. One way to stabilize such compounds is the application of high pressure. Here, through a direct reaction between Fe and N2 in a laser-heated diamond anvil cell, we synthesize three ironnitrogen compounds Fe3N2, FeN2 and FeN4. Their crystal structures are revealed by single-crystal synchrotron X-ray diffraction. Fe3N2, synthesized at 50 GPa, is isostructural to chromium carbide Cr3C2. FeN2 has a marcasite structure type and features covalently bonded dinitrogen units in its crystal structure. FeN4, synthesized at 106 GPa, features polymeric nitrogen chains of [N42−]n units. Based on results of structural studies and theoretical analysis, [N42−]n units in this compound reveal catena-poly[tetraz-1-ene-1,4-diyl] anions.

Strong compositional-dependent elastic properties have been observed theoretically and experimentally in Ti1−xAlxN alloys. The elastic constant, C11, changes by more than 50% depending on the Al-content. Increasing the Al-content weakens the average bond strength in the local octahedral arrangements resulting in a more compliant material. On the other hand, it enhances the directional (covalent) nature of the nearest neighbor bonds that results in greater elastic anisotropy and higher sound velocities. The strong dependence of the elastic properties on the Al-content offers new insight into the detailed understanding of the spinodal decomposition and age hardening in Ti1−xAlxN alloys.

In this study we discuss the performance of the special quasirandom structure (SQS) method in predicting the elastic properties of B1 (rocksalt) Ti 0.5 Al 0.5 N alloy. We use a symmetry-based projection technique, which gives the closest cubic approximate of the elastic tensor and allows us to align the SQSs of different shapes and sizes for a comparison in modeling elastic tensors. We show that the derived closest cubic approximate of the elastic tensor converges faster with respect to SQS size than the elastic tensor itself. That establishes a less demanding computational strategy to achieve convergence for the elastic constants. We determine the cubic elastic constants (C ij ) and Zener's type elastic anisotropy (A) of Ti 0.5 Al 0.5 N. Optimal supercells, which capture accurately both the configurational disorder and cubic symmetry of elastic tensor, result in C 11 = 447 GPa, C 12 = 158 GPa, and C 44 = 203 GPa with 3% of error and A = 1.40 with 6% of error. In addition, we establish the general importance of selecting proper SQS with symmetry arguments to reliably model elasticity of alloys. We suggest the calculation of nine elastic tensor elements: C 11 , C 22 , C 33 , C 12 , C 13 , C 23 , C 44 , C 55 , and C 66 , to analyze the performance of SQSs and predict elastic constants of cubic alloys. The described methodology is general enough to be extended for alloys with other symmetry at arbitrary composition.