Mixed-valence manganites with the ABO3 perovskite structure display a variety of magnetic and structural transitions, dramatic changes of electrical conductivity and magnetoresistance effects. The physical properties vary with the relative concentration of Mn3+ and Mn4+ in the octahedral corner-sharing network, and the proportion of these two cations is usually changed by doping the trivalent large A cation (for example, La3+) with divalent cations. As the dopant and the original cation have, in general, different sizes, and as they are distributed randomly in the structure, such systems are characterized by local distortions that make it difficult to obtain direct information about their crystallographic and physical properties. On the other hand, the double oxides of formula AA'3Mn4O12 contain a perovskite-like network of oxygen octahedra centred on the Mn cations, coupled with an ordered arrangement of the A and A' cations, whose valences control the proportion of Mn3+ and Mn4+ in the structure. The compound investigated in this work, (NaMn3+(3))(Mn3+(2)Mn4+(2))O12, contains an equal number of Mn3+ and Mn4+ in the octahedral sites. We show that the absence of disorder enables the unambiguous determination of symmetry, the direct observation of full, or nearly full, charge ordering of Mn3+ and Mn4+ in distinct crystallographic sites, and a nearly perfect orbital ordering of the Mn3+ octahedra.

We measured the density of vibrational states (DOS) and the specific heat of various glassy and crystalline polymorphs of SiO 2 . The typical (ambient) glass shows a well-known excess of specific heat relative to the typical crystal (α-quartz). This, however, holds when comparing a lower-density glass to a higherdensity crystal. For glassy and crystalline polymorphs with matched densities, the DOS of the glass appears as the smoothed counterpart of the DOS of the corresponding crystal; it reveals the same number of the excess states relative to the Debye model, the same number of all states in the low-energy region, and it provides the same specific heat. This shows that glasses have higher specific heat than crystals not due to disorder, but because the typical glass has lower density than the typical crystal. DOI: 10.1103/PhysRevLett.112.025502 PACS numbers: 63.20.-e, 07.85.-m, 76.80.+y The low-temperature thermodynamic properties of glasses are accepted to be anomalously different from those of crystals due to the inherent disorder of the glass structure. At temperatures of ∼10 K, the specific heat of glasses shows an excess relativetothatofthecorrespondingcrystals.Theexcessspecific heat is related to a distinct feature in the spectrum of the atomic vibrations: At frequencies of ∼1 THz, glasses exhibit an excess of states above the Debye level of the acoustic waves, the socalled "boson peak." The excess of specific heat and the boson peak are universally observed for all glasses and by all relevant experimental techniques. However, the results still do not converge to a unified answer to how disorder causes these anomalies.Themajorityofthemodelsexplainthebosonpeakbyappealing tovarious glass-specific features. Theseincludelow-energy optical modes [1], onset of mechanical instability related to saddle points in the energy landscape [2] or to jamming [3][4][5], local vibrationalmodes of clusters [6] or locally favoured structures [7], librations [8] or other coherent motions [9] of molecular fragments, crossover of local and acoustic modes [10], quasilocal vibrations of atoms in an anharmonic potential [11], broadening of vibrational states in the Ioffe-Regel crossover regime [12], spatial variation of the elastic moduli [13], breakdown of the continuum approximation [14,15], and topologically diverse defects [16], to cite the most important ones.Alternatively, the boson peak is identified as the counterpart of the acoustic van Hove singularities of crystals, i.e., explained by the piling up of the vibrational states of the acousticlike branches near the boundary of the pseudoBrillouin zone [17][18][19][20].Diverging in explanations of the boson peak, all models agree that the excess states and the excess specific heat of

The BiMnO3 perovskite is a very interesting multiferroic material that, once synthesized at high pressure and high temperature, survives as a metastable phase at ambient conditions. We investigated ceramic samples prepared in different conditions (temperature, pressure, and composition), and the existence of polymorphism at room temperature was clearly evidenced by electron diffraction and high-resolution electron microscopy in all the samples. A new polymorph, characterized by a different distortion of the perovskite basic cell, was found to coexist as a minor phase with the well-known C2 monoclinic form. The new polymorph, which can be described by a triclinic (pseudorhombohedral) superstructure with a = 13.62 Å, b = 13.66 Å, c = 13.66 Å, α = 110.0°, β = 108.8°, and γ = 108.8°, is mostly segregated at the grain surface. Magnetic characterizations revealed for this second form a critical temperature of 107 K, a few degrees above the ferromagnetic transition of the monoclinic C2 form measured at 99 K. The new phase disappears by reheating the samples at ambient pressure, suggesting the idea of a higher energy polymorph, which kinetically converts in the usual phase once a sufficient temperature has been achieved.

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Raman, Brillouin light, and x-ray scattering measurements have been carried out to characterize the low-frequency vibrational dynamics of the SiO(2) glass as function of its density. The obtained results demonstrate that while the distribution of the low-frequency states in the boson peak range is conserved under densification, these modes do not shift as a function of density as the acoustic modes do. The clear difference between the behavior of the vibrational states in the Boson peak range and that of the acoustic modes, could be explained considering the contribution of specific nonacoustic modes (tetrahedra rotation

The vibrational dynamics of a permanently densified silica glass is compared to the one of an -quartz polycrystal, the silica polymorph of the same density and local structure. The combined use of inelastic x-ray scattering experiments and ab initio numerical calculations provides compelling evidence of a transition, in the glass, from the isotropic elastic response at long wavelengths to a microscopic regime as the wavelength decreases below a characteristic length of a few nanometers, corresponding to about 20 interatomic distances. In the microscopic regime the glass vibrations closely resemble those of the polycrystal, with excitations related to the acoustic and optic modes of the crystal. A coherent description of the experimental results is obtained assuming that the elastic modulus of the glass presents spatial heterogeneities of an average size a $ =2. DOI: 10.1103/PhysRevLett.110.185503 PACS numbers: 63.50.Lm, 62.30.+d, 62.65.+k, 64.70.ph Amorphous solids lack the long-range translational periodicity of crystalline materials. Nevertheless, their structure presents a residual order on the short and medium ranges [1]. At short distances the structure can be characterized in terms of interatomic distances and bond-angles distribution. The medium-range order extends typically over a length D $ 2=ÁQ 0 of a few ($ 5) interatomic distances, as indicated by the width ÁQ 0 of the first sharp diffraction peak in the static structure factor, SðQÞ. The length scale of the nanometer is believed to be the relevant one to understand the phenomenology of the glass transition. In fact, close to the dynamical arrest, the atomic motion of a supercooled liquid is characterized by nanometer-sized regions where the molecules move cooperatively [2][3][4][5][6][7]. Recent numerical simulation studies [8][9][10] have also given some evidence of the presence of static correlation lengths of a size comparable to the dynamical correlations, by investigating either subtle structural order parameters [8] or point to set correlations [9,10]. However these quantities are not easily accessible experimentally, because they are not revealed by standard two-points correlation functions, such as the SðQÞ. Thereby a detailed description of the medium-range order in glasses is still missing.Only recently, the development of new experimental probes has given some evidence of the presence of atomic regions of nanometric size in a few amorphous materials. Local symmetries in a colloidal suspension have been detected by means of a cross correlation analysis using coherent x rays [11]. Subnanoscale-ordered regions originating from atomic polyhedra have also been detected in a metallic glass employing an electron nanoprobe supported by an ab initio molecular dynamics simulation [12]. On a similar glass a wide spatial distribution of the elastic modulus, on the length scale of the nanometer, has been detected by means of atomic force acoustic microscopy [13]. Here we employ an alternative way to gather information on the structure of the canon...

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The multiferroic perovskite BiMnO3, synthesized under high-pressure conditions, decomposes if heated at room-pressure in the temperature range of 500−650 °C. Comparative studies by high-temperature X-ray diffraction, electron diffraction, thermal analysis, and magnetic investigation revealed the existence of a complex pathway to decomposition, depending on the heating rate, pressure, and atmosphere that involves different metastable phases. In particular the as-prepared monoclinic phase (I) transforms to a second monoclinic form (II) at 210 °C and then to an orthorhombic phase (III) at 490 °C. These phase transitions, fast and reversible, occur on heating with a drop in volume and are moved at higher temperatures when pressure is decreased. The transition from II to III, typically observed in inert atmosphere, can be detected also in air when the heating rate is kept sufficiently high. When III is heated in an oxygen-containing atmosphere a slow irreversible transition to variants IV and then V takes place with kinetics depending on temperature, heating rate, and oxygen partial pressure. Both IV and V are oxidized ferromagnetic phases containing Mn4+ characterized by a modulated structure based on fundamental triclinic perovskite cells. Their magnetic behavior shows a strong analogy with thin films of BiMnO3, suggesting for the latter an oxidized nature and for the former a possible multiferroic behavior.