X-ray diffraction, Raman spectroscopy, and electrical conductivity measurements of molybdenum disulfide MoS(2) are performed at pressures up to 81 GPa in diamond anvil cells. Above 20 GPa, we find discontinuous changes in Raman spectra and x-ray diffraction patterns which provide evidence for isostructural phase transition from 2H(c) to 2H(a) modification through layer sliding previously predicted theoretically. This first-order transition, which is completed around 40 GPa, is characterized by a collapse in the c-lattice parameter and volume and also by changes in interlayer bonding. After the phase transition completion, MoS(2) becomes metallic. The reversibility of the phase transition is identified from all these techniques.

Recent developments in crystal structure prediction, in particular, the powerful evolutionary algorithm USPEX [1,2], enable reliable prediction of stable compounds formed by given elements. At normal conditions such calculations produce the well-known stable compounds: e.g., NaCl as the only compound of Na and Cl, or MgO as the only stable compound of Mg and O. At high pressures and in low-dimensional materials, unexpected phenomena have been predicted-then experimentally verified. I will discuss several recent examples: 1. Discovery of two new stable high-pressure compounds of helium, Na2He and Na2HeO (Na2He has been synthesized experimentally) [3]. This discovery has implications for both fundamental chemistry and planetary sciences. 2. Formation of new stable sodium chlorides: Na3Cl, Na2Cl, Na3Cl2, NaCl3, NaCl7 [4], Na4Cl3 [5], and a large number of new stable potassium chlorides [6]. These predictions were verified experimentally [5,6] and are still not fully understood. 3. New stable magnesium oxides: Mg3O2 and MgO2 [7] and MgO3 [8], and silicon oxides SiO and SiO3 [8]. Among these predictions, stability of MgO2 has already been experimentally confirmed [9]. These predictions may have implications for planetary chemistry. 4. USPEX-based prediction of the Cui group [10] and experimental verification of Eremets group [11] of a new high-temperature superconductor-cubic H3S. This discovery opens new hopes for room-temperature superconductivity. 5. Prediction [12] that dominant silicon oxide nanoparticles at normal conditions (ambient P-T, and normal air) will be oxygen-enriched and magnetic: e.g. Si7O19. This may explain well-documented carcinogenic activity of fine silica dust. Future avenues for explanation and generalization of these phenomena will be discussed. 1] Oganov A.R. et al, J.