Molybdenum borides are currently raising great expectations for superhard materials, but their crystal structures and mechanical behaviors are still under discussion. Here, we report an unexpected reduction of stability and hardness from porous hP16-MoB3 and hR18-MoB2 to dense hP20-MoB4 and hR21-Mo2B5, respectively. Furthermore, we demonstrate that this anomalous variation has its electronic origin. These findings not only manifest that the long-recognized hP20-MoB4 (hP3-MoB2) and hR21-Mo2B5 should be hP16-MoB3 and hR18-MoB2, respectively, but also challenge the general design principle for ultrahard materials only pursuing the dense transition-metal borides with high boron content.

A systematic examination of structural phase transitions and mechanical properties for TiO2 under high pressure has been performed by using first‐principles calculations. First, we show that the orthorhombic Pca21 structure becomes stabilize at certain pressure and temperature, whereas the cubic fluorite and pyrite structures are not energetically viable in the whole pressure range of 0–200 GPa. These findings support that the experimentally assumed cubic TiO2 should be the Pca21‐type TiO2. Secondly, our calculated equations of state for various TiO2 polymorphs are consistent with previous experimental and theoretical results. The only exception is the baddeleyite phase for which we find a significantly lower bulk modulus of 149 GPa than the measured value 290–304 GPa. Finally, our calculations reveal that the recently synthesized Fe2P‐type TiO2 exhibits semiconducting features and has the potential to be a superhard material under ultrahigh pressure. It is shown that the high pressure could open a valid avenue for new hard or superhard materials.

We report a drastic reduction of hardness of about 61% from WB3 to WB4. The three-dimensional covalent network consisting of boron honeycomb planes interconnected with strong zigzag W-B bonds underlies the high hardness of WB3. Despite the strong intralayer and interstitial B-B bonds, the interlayer B-B nonbonding and the considerably weak zigzag W-B bonding allow the layers of WB4 to cleave readily, which results in the anomalous softening of WB4. The results provide robust evidence that the highest boride of tungsten, which is characterized experimentally by an inexpensive superhard material, should be stoichiometric WB3, not WB4.

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