Papers by Author: Xu Guang Zheng

Abstract: Factor group analysis on the deformed pyrochlore-structure hydroxyl transition metal halides M₂(OH)₃X (M = Co, Fe and Mn; X = Cl, Br and I) with the crystal symmetry ≡ D_3d⁵ of space group No. 166 for their infrared (IR) and Raman spectroscopic studies are, for the fist time to the best of our knowledge, reported. Through a standard factor group analysis, we predict that there are 17 IR active normal modes (7A_2u+ 10E_u) and 12 Raman active modes (5A_1g + 7E_g), caused by five atomic site groups [HOM₃], [OM₃], [MO₄X₂], [ClM₃] and [MO₆], where the black body letters represent the site atoms. In addition we figure out all the atomic displacement patterns of 12 Raman active normal modes, and point out these results are completely same as those of the famous geometric frustration material Herbertsmithite ZnCu₃(OH)₆C_l2.

Abstract: The low temperature Raman spectroscopic (4000-95 cm^–1) investigation on the hydroxyl cobalt chloride Co₂(OH)₃Cl and deuteroxyl cobalt chloride Co₂(OD)₃Cl polycrystalline samples has been performed from the room temperature down to 4 K closing to the liquid Helium temperature. Lineshapes of Raman mode E_g (about 130 cm^–1 assigned to the normal symmetrically bending vibration mode of the [ClCo₃] group for both kinds of samples) are found to be apparently asymmetric Fano lineshapes when the temperatures are below 230 K. This phenomenon is preliminarily attributed to the effect of electron-phonon coupling and a continuum of electronic states centered at about 16 meV with an upper limit 18.5 meV is suggested in this report.

Abstract: Through ten years physical investigation on the geometrically frustrated coordination compound series basic (hydroxyl and deuteroxyl) transition-metal halides M₂(OH/D)₃X (M=Mn, Fe, Co, Ni and Cu; X=Cl, Br and I), we have determined the existence of four crystal structures, whose space groups belong to Nos. 11, 14, 62 and 166, respectively, in this series till now. After studying their Raman spectra, we firmly believe that a nonnegligible hydrogen bond (H-bond) exists in every material. Considering carefully their local lattice environments of the basic groups, we find a new kind, to the best of our knowledge, of H-bond [(OH)₃···M] and nominate it as the trimeric H-bond, and further confirm three types of symmetric structures, which have totally four subtypes of trimeric H-bonds so far. More two subtypes are predicted to exist in nature. This result may provide a new interesting subject for quantum chemists and material physicists.

Abstract: The low temperature (T) Raman spectroscopic study on the geometrically frustrated clinoatacamite γ-Cu₂(OH)₃Cl is reported. By measuring the T-dependent spectral profiles and representative modes, we have obtained auxiliary evidence of successive magnetic transitions near T_c1 =18.1 K and T_c2 =6.4 K determined accurately by the magnetic experiments in the Raman band frequencies and peak widths of the representative modes for magnetic research conclusions and observed a pronounced Raman spectroscopy background featuring a broad continuum at all Ts. A quantitative analysis reveals spin fluctuations on a picosecond time scale in the intermediate phase.

Abstract: Basic copper chloride Cu2(OH)3Cl and Cu2(OD)3Cl polycrystalline sample were successfully synthesized in the same single phase of clinoatacamite-structure, and the latter’s mid-infrared absorption spectra (4000-400cm–1) and Raman spectra (4000-95cm–1) were first, to the best of our knowledge, measured respectively by FTIR spectroscopy and Micro-Raman spectroscopy to study the corresponding relationship between their spectral properties and crystal structure. Through the comparative analysis of the four spectra we definitely assign or tentatively suggest the vibration modes of OH/D groups in the trimeric hydrogen bond environment, (H/DO)–Cu–(OH/D), Cl–Cu–Cl. These results can be propitious to their low temperature spectral properties which must help to understand the underlying physics of their exotic geometric frustration phenomenon.

Abstract: The geometrically frustrated material-deuteroxyl cobalt chlorides Co2(OD)3Cl sample was first, to the best of our knowledge, successfully synthesized in the same single phase as the famous Co2(OH)3Cl, and their mid-infrared absorption spectra (4000-400cm–1) and Raman spectra (4000-95 cm–1) are first, to the best of our knowledge, measured respectively by FTIR spectroscopy and Micro-Raman spectroscopy to study the corresponding relationship between their spectral properties and crystal structure. Through the comparative analysis of the four spectra we definitely assign or tentatively suggest the vibration modes of hydroxyl/deuteroxyl groups in the trimeric hydrogen bond environment, (OH/D)–Co–(OH/D), and (H/D3···Cl)–Co–(Cl···H/D3). These results can be propitious to the low temperature spectral properties of Co2(OH)3Cl and Co2(OD)3Cl which must help to understand the underlying physics of their exotic geometric frustration phenomenon.

Abstract: The mid-IR and Raman spectra of geometrically frustrated atacamite hydroxyl copper chloride are measured by FTIR spectroscopy and HR800 Raman spectroscopy, respectively. The microscopic origins of bands within the functional group and the fingerprint regions are suggested by the known crystal structure parameters. Five distinct regions involved in hydroxyl stretching, overtones, hydroxyl deformation, Cu-O stretching, O–Cu–O symmetric stretching and O–Cu–O bending modes, pyramid-structure Cl≡Cu3 vibration modes, lattice modes, respectively.