The sequence of high‐pressure phase transitions α→β→γ in olivine is traditionally used as a model for seismic velocity variations in the 200‐ to 650‐km‐depth interval in a mantle of peridotitic bulk composition. It has been proposed that the observed seismic velocity increase at 400‐km depth is too sharp and of too small a magnitude to be attributable to the α→β phase change and that the upper mantle must thus be chemically stratified, with the 400‐km discontinuity being due either to a combination of phase changes in a layer of pyroxene/garnet‐rich “piclogite” composition or to a chemical boundary between such a piclogite layer and an overlying peridotitic layer. Using available calorimetric, thermoelastic, and synthesis data (and their associated experimental uncertainties), we have derived internally consistent high‐pressure phase relations for the Mg₂SiO₄‐Fe₂SiO₄ join. We find that the divariant transition α→α+β→β, which is generally regarded as occurring over a broad depth interval for mantle olivine compositions, is, in fact, extremely sharp. The seismic discontinuity corresponding to the α→α+β→β transition in (Mg_0.9Fe_0.1)₂SiO₄ should occur over a depth interval (isothermal) of about 6 km at a depth of approximately 400 km; the sharpness of this transition is quite insensitive to uncertainties in the constraining calorimetric, thermoelastic, and synthesis data. In addition, we have computed seismic velocity profiles for a model mantle consisting of pure olivine of (Mg_0.9Fe_0.1)₂SiO₄ composition. Comparison of these computed profiles to those derived from recent seismic studies indicates that the magnitude of the observed velocity increase at 400‐km depth is consistent with a mantle transition zone composed of about 60–70% olivine. We conclude that there is no need to infer the existence of pyroxene/garnet‐rich compositions, such as eclogite or “piclogite,” in the transition zone, since an upper mantle of homogeneous olivine‐rich peridotitic composition is consistent with the available seismic velocity data.

The pressure-volume equation of state (EoS) of single-crystal MgO has been studied in diamond-anvil cells loaded with helium to 118 GPa and in a non-hydrostatic KCl pressure medium to 87 GPa using monochromatic synchrotron X-ray diffraction. A third-order Birch-Murnaghan fit to the non-hydrostatic P-V data (KCl medium) yields typical results for the initial volume, V₀ = 74.698(7) ų, bulk modulus, K_T0 = 164(1) GPa, and pressure derivative, K′ = 4.05(4), using the non-hydrostatic ruby pressure gauge of Mao et al. (1978). However, compression of MgO in helium yields V₀ = 74.697(6) ų, K_T0 = 159.6(6) GPa, and K′ = 3.74(3) using the quasi-hydrostatic ruby gauge of Mao et al. (1986). In helium, the fitted equation of state of MgO underdetermines the pressure by 8% at 100 GPa when compared with the primary MgO pressure scale of Zha et al. (2000), with K_T0 = 160.2 GPa and K′ = 4.03. The results suggest that either the compression mechanism of MgO changes above 40 GPa (in helium), or the ruby pressure gauge requires adjustment for the softer helium pressure medium. We propose a ruby pressure gauge for helium based on shift of the ruby-R₁ fluorescence line (Δλ/λ ₀) and the primary MgO pressure scale, with P (GPa) = A/B{[1 + (Δλ/λ₀)]B − 1}, where A is fixed to 1904 GPa and B = 10.32(7).

Diamond is a wide-bandgap semiconductor possessing exceptional physical and chemical properties with the potential to miniaturize high-power electronics. Whereas boron-doped diamond (BDD) is a well-known p-type semiconductor, fabrication of practical diamond-based electronic devices awaits development of an effective n-type dopant with satisfactory electrical properties. Here we report the synthesis of n-type diamond, containing boron (B) and oxygen (O) complex defects. We obtain high carrier concentration (∼0.778 × 10²¹ cm⁻³) several orders of magnitude greater than previously obtained with sulfur or phosphorous, accompanied by high electrical conductivity. In high-pressure high-temperature (HPHT) boron-doped diamond single crystal we formed a boron-rich layer ∼1–1.5 μm thick in the {111} surface containing up to 1.4 atomic % B. We show that under certain HPHT conditions the boron dopants combine with oxygen defects to form B–O complexes that can be tuned by controlling the experimental parameters for diamond crystallization, thus giving rise to n-type conduction. First-principles calculations indicate that B₃O and B₄O complexes with low formation energies exhibit shallow donor levels, elucidating the mechanism of the n-type semiconducting behavior. Significance Diamond is a uniquely attractive wide-bandgap semiconductor for future electronic devices where its remarkable physical properties may enable switches, transistors, and diodes for extreme applications. Whereas p-type semiconducting diamond is well developed with boron doping, the synthesis of n-type diamond, required to complete purely diamond-based electronics, has been a great challenge in materials science. We report the creation of n-type diamond via a new type of defect complexes of boron and oxygen that can be tuned by controlling the experimental parameters for diamond crystallization, leading to a shallow donor state with high carrier concentration that is several orders of magnitude higher than achieved by sulfur or phosphorus doping. The results provide a new strategy for producing n-type diamond-based devices.