Phase transitions and energetic properties of Li2Mg(NH)2 with different crystal structures are investigated by experiments and first-principles calculations. The Li2Mg(NH)2 with the primitive cubic and orthorhombic structure is obtained by dynamically dehydrogenating a Mg(NH2)2-2LiH mixture up to 280 °C under an initial vacuum and 9.0 bars H2, respectively. It is found that the obtained orthorhombic Li2Mg(NH)2 is converted to a primitive cubic structure as the dehydrogenation temperature is further increased to 400 °C or performed by a 36 h of high-energetic ball milling. Moreover, the primitive cubic phase can be converted to an orthorhombic phase after heating at 280 °C under 9.0 bars H2 for 1 h. Thermodynamic calculations show that the orthorhombic phase is the ground state structure of Li2Mg(NH)2. The mechanism for phase transitions of Li2Mg(NH)2 is also discussed from the angle of energy.

SUMMARYA new curved quadrilateral composite shell element using vectorial rotational variables is presented. An advanced co-rotational framework defined by the two vectors generated by the four corner nodes is employed to extract pure element deformation from large displacement/rotation problems, and thus an element-independent formulation is obtained. The present line of formulation differs from other corotational formulations in that (i) all nodal variables are additive in an incremental solution procedure, (ii) the resulting element tangent stiffness is symmetric, and (iii) is updated using the total values of the nodal variables, making solving dynamic problems highly efficient. To overcome locking problems, uniformly reduced integration is used to compute the internal force vector and the element tangent stiffness matrix. A stabilized assumed strain procedure is employed to avoid spurious zero-energy modes. Several examples involving composite plates and shells with large displacements and large rotations are presented to testify to the reliability, computational efficiency, and accuracy of the present formulation.

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