Milko N. Iliev

Raman spectra of superconducting and impurity phases in Bi-Ca-Sr-Cu-O ceramics

Abstract Micro-Raman technique in conjunction with electron microanalysis were employed for identification of Raman spectra of superconducting and impurity phases in Bi-Ca-Sr-Cu-O ceramics obtained by three different technologies. Two of the main impurity phases are (Ca0.9Sr0.1)2CuO3 and (Ca0.6Sr0.4)Cu1.75O3 and their Raman spectra are very similar in the region below 400 cm−1. Another impurity phase is Bi2Ca1Sr3Ox (with slight variations of Bi-Ca-Sr composition) which displays strong Raman scattering at 615–622 cm−1 and a sharp peak at 90 cm−1. The Raman spectrum of the superconducting Bi2Ca1Sr2Cu2Ox phase is characterized by a strong peak at 635 cm−1 and a weak feature near 460 cm−1. The Raman spectra from microcrystals of Bi-Ca-Sr-Cu-O composition close to 3233, 3223, 2142 and 4123 have common features with the one of Bi2Ca1Sr2Cu2Ox. Some of them, e.g. 2142, exhibit almost continuous scattering below 620 cm−1, which may be caused by strong disorder thus displaying the one-phonon density of states.

Optical phonons probe of the SrLaAlO4 crystal structure

Abstract We present a study of the vibrational properties of SrLaAlO 4 based on Raman scattering and IR measurements of an oriented single crystal. All the four Raman and seven IR phonons allowed by symmetry for the tetragonal 14/mmm space group have been identified. The measured phonon frequencies in SrLaAlO 4 are compared with those calculated on the basis of the shell model. Besides the Raman allowed phonon peaks, a number of additional Raman structures appear in the measured spectra. Some of them have been related to defects or second-order scattering processes. Three peaks with well defined symmetries appeared in forbidden scattering geometries. The symmetry of these peaks suggests short range ordering of Sr and La over lattice sites of the same symmetry.

Raman scattering from monoalkali (Na-Sb and K-Sb), bialkali (Na-K-Sb) and multialkali (Na-K-Sb-Cs) photocathodes

Abstract The Raman spectra of monoalkali (Na-Sb and K-Sb), bialkali (Na-K-Sb) and multialkali (Na-K-Sb-Cs) photocathodes were measured at room temperature. Three Raman lines were detected in Na-Sb at 45 cm -1 , 128 cm -1 and 187 cm -1 . The latter line was assigned to A 1g vibrations of sodium atoms at the (f) sites of the hexagonal ( D 4 6h ) lattice of Na 3 Sb. The only Raman-active mode ( F 2g ) in cubic ( O 5 h ) K 3 Sb was found at 145 cm -1 . The Raman line at 180–185 cm -1 in the spectra of the bialkali and multialkali photocathodes was assigned to the F 2g vibration of sodium atoms at the (c) sites of the cubic Na 2 KSb lattice. It was established that in the latter type of photocathode a definite amount of the cubic K 3 Sb phase is also present.

Polarized Raman scattering from Bi-based HTS single crystals and its relation to the lattice dynamics of Y-123 compounds

Abstract The polarized Raman spectra of Bi-HTS microcrystals with defined orientation of the c -axis and of different Bi-Ca-Sr-Cu composition (including close to 2122 and 2223) were measured at room temperature by means of the Raman microprobe technique. The phonon lines at 635 cm -1 (A 1g ), 460 cm -1 (B 1g , near 300 cm -1 (A 1g ) and at 115 cm -1 (A 1g ) appear for alll microcrystals, whereas the position of the relatively strong (A 1g ) line in the low frequency region depends on the crystal. The Raman lines are assigned to certain lattice vibrations on the basis of comparison of the Raman spectra of Y-123 and Bi-HTS and of the structural data for Y-123 and Bi-2122. Earlier assignments of the Raman lines in the orthorhombic and tetragonal Y-123 are revised.

Raman and Mössbauer study of the pseudo-orthorhombic-to-tetragonal phase transition in YBa2(Cu1-xFex)3O7-δ(0.02≤x≤0.15)

Abstract The polarized Raman spectra from microcrystals of YBa 2 (Cu 1− x Fe x ) 3 O 7− δ (0.02⩽ x ⩽0.15) were studied in various scattering configurations allowing one to follow the variations with x of both diagonal (A g ) and non-diagonal (B 2g and B 3g ) Raman modes. It was found that the splitting of the strongest in intensity B 2g , B 3g Raman pair at 210 and 300 cm −1 associated with O(4) vibrations along a and b , respectively, decreases slightly with x , thus indicating that in a microscopic scale the structure remains orthorhombic over the whole substitutional range. The Mossbauer spectra for x =0.05, 0.10, and 0.15 showed a superlinear increase of the number of five-fold oxygen-coordinated Fe-atoms at the Cu(1)-sites. This is consistent with the assumption that Fe-clusters are formed along the microtwin boundaries at higher x . In this sense YBa 2 (Cu 1− x Fe x ) 3 O 7− δ could be considered as a two-phase system. The observed splitting of the A g Raman mode of Ba at x ⩾0.07 supports such an assumption. The Fe substitution increases the local disorder thus including additional Raman scattering of one-phonon density-of-states origin with a maximum at 580 cm −1 .

Raman study of hydrogenated RBa2Cu3O7−δ (R = Y, Gd)

Abstract Single crystal Raman spectra of various scattering geometries were obtained from ceramic H1.0YBa2Cu3O6.7 and H1.0GdBa2Cu3Oy using the microprobe technique. Apart from the five prominent Ag Raman lines being characteristic for YBa2Cu3O7, three more lines at 210 cm−1, 248 cm−1 and 610 cm−1 with exact Ag symmetry are observed in H1.0YBa2Cu3O6.7. The lowest frequency extra Raman lines are unambiguously attributed to IR vibrations ofY and Cu(1), respectively. The line at 610 cm−1 arises from oxygen vibrations but can not be conclusively assigned. Hydrogen occupies the interstitial sites around Cu(1) and Cu(2) (within the Y-plane) and perturbs the vibrations of its nearest-neighbours. The experimental results imply that the modes involving otherwise only odd parity vibrations become Raman active under hydrogen doping. The Raman scattering evidence is given that the hydrogen doping decreases the free holes concentration in YBa2Cu3O7−δ.

A Raman study of the structural properties of YBa2(Cu1 − xFex)3Oy

Abstract The effects of Fe-substitution of YBa 2 Cu 3 O y have been investigated by means of Raman scattering, X-ray diffraction, resistivity and susceptibility measurements. A series of samples of YBa 2 (Cu 1 − x Fe x ) 3 O y with different dopant concentration (0 ⩽ x ⩽ 0.15) has been prepared in two batches, the second set having undergone twice the heat and mechanical treatment used to produce the first batch. Considerable improvement in the superconducting transition temperature, T c , is obtained upon reprocessing. A phase transformation from orthorhombic to tetragonal symmetry is observed for x =0.05 from the X-ray measurements in agreement with previous work. Using a micro-Raman technique, all five A g vibrational modes have been measured and their dependence on Fe-concentration is analyzed. There are indications that iron substitutes for copper at both sites and that the structure is a mixture of orthorhombic and tetragonal microdomains for all x .

Polarised Raman scattering from orthorhombic and tetragonal GdBa2Cu3O7−δ single crystals: New evidence of chains-to-planes charge redistribution

Abstract Raman scattering spectra of various polarisation configurations have been obtained from as-grown single crystals of Gd-Ba 2 Cu 3 O 7−δ in both the orthorhombic (δ≈0.1) and tetragonal (0.7 g phonons at 118, 149, 331, 433 and 506 cm -1 , two more Raman lines at 205 and 302 cm -1 have been observed in cross-polarisation spectra of the orthorhombic phase exhibiting B 2g and B 3g symmetries respectively. There was no observation of respective nondiagonal E g symmetry modes in the tetragonal phase. The changes in the lineshapes and intensities for the same scattering configurations of the orthorhombic and tetragonal phases provide evidence for a strong influence of the free carriers on the scattering process. The experimental results are compatible with the concept of chains-to-planes charge transfer.

Destruction of non-superconducting YBa2Cu3O6.3 and PrBa2Cu3O6.8 due to hydrogenation: Raman scattering and X-ray diffraction study

Abstract The decomposition of nonsuperconducting tetragonal YBa2Cu3O6.3 and orthorhombic PrBa2Cu3O6.8 due to reaction with hydrogen was studied by means of optical microscopy, X-ray diffraction, Raman spectroscopy and SEM analysis on ceramic samples with nominal composition HxYBa2Cu3O6.3 (x=0, 0.3, 1 and x⪢1) and HxPrBa2Cu3O6.8 (x=0, 1, 2 and x⪢1). Among the products of decomposition BaO, CuO, Cu2O and Cu were unambigously identified. The picture is complicated by additional reaction between hydrogen and the products of degradation. The results of this study clearly show that contrary to the case of superconducting orthorhombic YBa2Cu3O7-δ hydrogen cannot be incorporated in either tetragonal Y-123 or orthorhombic Pr-123 crystals. Thus the ability to absorb hydrogen is related to the existence of free holes in the 123-system.

Polarized Raman spectra of superconducting and semiconducting YBa2Cu3Ox single crystals

Abstract The polarization properties of Raman scattering from single crystals of YBa2Cu3Ox are studied at room temperature for both the superconducting (x≊7) and semiconducting (x≊6.25) phases. The assignment of the phonon lines at 502, 435, 339, 148, 118 and 59 cm−1 (for the orthorhombic phase) and at 482, 450, 341, 208?, and 140 cm−1 (for the tetragonal phase) to certain lattice vibrations is discussed.