Superconductivity often emerges in the proximity of, or in competition with, symmetry breaking ground states such as antiferromagnetism or charge density waves (CDW) 1--5 . A number of materials in the cuprate family, which includes the high--transition--temperature (high--T c ) superconductors, show spin and charge density wave order 5--7 . Thus a fundamental question is to what extent these ordered states exist for compositions close to optimal for superconductivity. Here we use high--energy x--ray diffraction to show that a CDW develops at zero field in the normal state of superconducting YBa 2 Cu 3 O 6.67 (T c = 67 K). Below T c , the application of a magnetic field suppresses superconductivity and enhances the CDW. Hence, the CDW and superconductivity are competing orders in this typical high--T c superconductor, and high--T c superconductivity can form from a pre--existing CDW state. Our results explain observations of small Fermi surface pockets 8 , negative Hall and Seebeck effect 9,10 and the "T c plateau" 11 in this material when underdoped.Charge density waves in solids are periodic modulations of conduction electron density. They are often present in low dimensional systems such as NbSe 2 4 . Certain cuprate materials such as La 2--x--y Nd y Sr x CuO 4 (Nd--LSCO) and La 2--x Ba x CuO 4 , (LBCO) also show charge modulations that suppress superconductivity near x=1/8 6,7 . In some cases, these are believed to be unidirectional in the CuO 2 plane, and have been dubbed 'stripes' 2,3 . There is now a mounting body of indirect evidence that charge and/or spin density waves may be present at high magnetic fields in samples with high T c : quantum oscillation experiments on underdoped YBa 2 Cu 3 O y (YBCO) have revealed the existence of at least one small Fermi surface pocket 8,9 which may be created by a charge modulation 10 . More recently, NMR studies have shown a magnetic--field--induced splitting of the Cu2F lines of YBCO 12 . An important issue is the extent to which the tendency towards charge order exists in high--T c superconductors 2,3 .Here we report a hard (100 keV) X--ray diffraction study, in magnetic fields up to 17 T, of a de-twinned single crystal of YBa 2 Cu 3 O 6.67 (with ortho--VIII oxygen ordering 11,13 , T c = 67 K and p = 0.12 where p is the hole concentration per planar Cu). We find that a CDW forms in the normal state below T CDW ≈ 135 K. The charge modulation has two fundamental wavevectors q CDW = q 1 = (δ 1 , 0, 0.5) and q 2 = (0, δ 2 , 0.5), where δ 1 ≈ 0.3045(2) and δ 2 ≈0.3146(7). These give satellites of the parent crystal Bragg peaks at positions such as Q=(2±δ 1 , 0, 0.5). Although the satellite intensities have a strong temperature and magnetic field dependence, the CDW is not field-induced and is unaffected by field in the normal state. Below T c it competes with superconductivity, and a decrease of the CDW amplitude in zero field becomes an increase when superconductivity is suppressed by field. Figure 1a,g shows scans through the (2--δ 1 , 0, 0.5) and (0, 2--δ 2 , 0.5) p...

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For the first time a detailed structural model has been determined which shows how the lone-pairs of electrons are arranged relative to each other in a glass network containing lone-pair cations. High energy X-ray and neutron diffraction patterns of a very high lead content silicate glass (80PbO·20SiO2) have been used to build three-dimensional models using empirical potential structure refinement. Coordination number and bond angle distributions reveal structural similarity to crystalline Pb11Si3O17 and α- and β-PbO, and therefore strong evidence for a plumbite glass network built from pyramidal [PbO(m)] polyhedra (m ~ 3-4), with stereochemically active lone-pairs, although with greater disorder in the first coordination shell of lead compared to the first coordination shell of silicon. The oxygen atoms are coordinated predominantly to four cations. Explicit introduction of lone-pair entities into some models leads to modification of the local Pb environment, whilst still allowing for reproduction of the measured diffraction patterns, thus demonstrating the non-uniqueness of the solutions. Nonetheless, the models share many features with crystalline Pb11Si3O17, including the O-Pb-O bond angle distribution, which is more highly structured than reported for lower Pb content glasses using reverse Monte Carlo techniques. The lone-pair separation of 2.85 Å in the model glasses compares favourably with that estimated in α-PbO as 2.88 Å, and these lone-pairs organise to create voids in the glass, just as they create channels in Pb11Si3O17 and interlayer spaces in the PbO polymorphs.

The ammonium analogues of the high-pressure potassium-bearing silicate phases K-hollandite, K-Si-wadeite, K-cymrite, and phengite were synthesized in the system (NH 4 Ammonium in eclogite facies metasediments is mainly bound in micas and concentrations may reach up to a few thousand ppm. It can be stored to greater depths in high-pressure potassium silicates during ongoing subduction. This possibly provides an important mechanism for nitrogen and hydrogen transport into the deeper mantle.

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