Crystal and electronic structure of high temperature superconducting compound Y1−xCaxBa2Cu3Oy in the temperature interval 80–300 K

Accepted Manuscript Crystal and electronic structure of high temperature superconducting compound Y1xCaxBa2Cu3Oy in the temperature interval 80-300 K Svetlana G. Titova, Alexey V. Lukoyanov, Stepan V. Pryanichnikov, Lubov A. Cherepanova, Alexander N. Titov PII:

S0925-8388(15)31575-9

DOI:

10.1016/j.jallcom.2015.11.019

Reference:

JALCOM 35874

To appear in:

Journal of Alloys and Compounds

Received Date: 29 June 2015 Revised Date:

3 November 2015

Accepted Date: 3 November 2015

Please cite this article as: S.G. Titova, A.V. Lukoyanov, S.V. Pryanichnikov, L.A. Cherepanova, A.N. Titov, Crystal and electronic structure of high temperature superconducting compound Y1xCaxBa2Cu3Oy in the temperature interval 80-300 K, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.11.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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T < T1  T > T2

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T1< T
T1

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T2

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b (105 K-1)

5

-5

-10 100

150

200

T>T2 T< T1 T1 < T< T2

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Densities of states (st./eV f.u.)

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T, K

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-1

0

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Energy (eV)

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Crystal and electronic structure of high temperature superconducting ACCEPTED MANUSCRIPT compound Y1-xCaxBa2Cu3Oy in the temperature interval 80-300 K

Svetlana G. Titovaa,*, Alexey V. Lukoyanov,b,c, Stepan V. Pryanichnikova,

a

Institute of Metallurgy Urals Division of Russian Academy of Sciences, 620016, Еkaterinburg, Russia

Institute of Metal Physics of Russian Academy of Sciences, 620137, Еkaterinburg, Russia

c

Ural Federal University, 620002, Еkaterinburg, Russia

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b

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Lubov A. Cherepanovaa, Alexander N. Titovb,c

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Abstract

For Y1-хСахBa2Cu3Oy with varying oxygen and calcium content, the change of crystal structure at cooling from room temperature to 80 K has been investigated. The main change is associated with a shift of apical oxygen atoms. Using determined unit cell parameters as function of temperature, the coefficients of linear thermal expansion have been calculated as

α X = 1 / X ⋅ ( dX / dT ) , where Х= a, b, c – unit cell dimensions. For the compound without

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calcium all α X values demonstrate anomalous behavior such as growing at cooling in temperature interval T1 ÷ T2 ~150 ÷ 225 K. The calculation of electronic structure at temperatures above, below and within this interval shows the appearance of the electronic states

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density peak for Ba and O4 atoms. It is explained by the localization of charge carriers with the participation of the lattice distortion in a form of apical Cu-O bond compression. As the material

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Y0.9Са0.1Ba2Cu3Oy regardless of the oxygen content possess low and constant coefficient of thermal expansion along all crystallographic directions, that makes this material suitable for superconducting films and other composites.

Keywords: Superconductors, crystal structure, electronic band structure, thermal expansion

*

Corresponding author at: Institute of Metallurgy Urals Division of Russian Academy of Sciences, 620016, Еkaterinburg, Russia. Tel. +7 343 2329075; fax: +7 343 2679186. E-mail address: [email protected] (S.G. Titova)

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1. Introduction

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For understanding the mechanism of superconductivity and other properties of high temperature superconductors, the information about their state determined by degree of doping is important. At present, the existence of the pseudogap state [1] at temperature T* which represents the destruction of the Fermi surface in the vicinity of “hot-spots” (close to nodal direction) has firmly been established. Some authors explain that strong scattering of correlated

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electrons on short-range antiferromagnetic fluctuations are responsible for the opening of the pseudogap [2]. From the other hand, for number of HTSC cuprates with use of complementary set of such experimental methods like scanning-tunneling microscopy, angle-resolved photoemission spectroscopy, resonant elastic and inelastic X-ray scattering [3-6], high-energy X-

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ray scattering in applied magnetic field [7], soft and hard X-ray diffraction [8, 9] an evidences for existence of charge-ordered state in both underdoped and optimally doped cuprates and its intimate connection to the pseudogap regime were provided. Additionally, it was shown that

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intensity of Bragg peaks caused by charge-ordered state in form of charge density wave (CDW) tracks the intensity of the low-energy peaks caused by spin fluctuations [9], it reveals intimately entanglement of different order parameters in cuprates. It is known that T* is sensitive to isotopic composition. This effect was manifested most brightly for HoBa2Cu4O8 and Er2Ba4Cu7O15 at underdoped state and substitution of 16O by 18О [10]. The pseudogap opens at 170 K and 220 K for samples with 16O and 18О, respectively, which corresponds to the isotopic

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shift of ∆Т*≈ 50 K. Such a significant isotope effect notes about clear contribution of phonon subsystem to formation of a pseudogap state as well as other properties of HTSC-materials. When partial substitution Cu/Cd was used, T* had been shifted to lower temperatures with

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increasing Cd concentration [11], it was explained by heavier cadmium suppression of the density of phonons which in turn suppress T*. The temperature T* strongly depends on doping state p, decreases with p rising. On the

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other hand, using acoustic methods (ultrasound attenuation) for various HTSC materials, the anomalies in form of maxima of the attenuation coefficient were detected. Their temperatures weakly depended on the doping state (see, for example, [12] and references cited there, as well as earlier results [13-18]). Low temperature X-ray and neutron powder diffraction studies of Biand Hg-based HTSC cuprates reveal three different structure anomalies at temperatures T0~Tc+15 K, T1~160 K and T2~260 K. The temperature of the two last ones does not almost depend on either doping state or even a type of material [19] that denotes that local interaction is responsible for these anomalies. For a row of HTSC compounds the compression of apical Cu-O bond in the temperature interval T1-T2 was found [19] and interpreted as a result of charge carrier localization at participation of lattice deformation in temperature interval ~160-260 K. 2

The independence of this interval from the charge carrier concentration and even chemical

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composition of HTSC compound confirms this interpretation. Because of independence of this temperature interval from doping state, this effect cannot be associated with pseudogap opening. The thermal evolution of crystal structure of cuprate materials was studied by diffraction methods, both for powders and de-twinned single crystals. The obtained results diverge significantly. For example, the authors in [20], in contrary with data [19, 21-22], have found extremely weak structure features around 150 and 250 K, and only for one cell dimension

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a, although the research was performed for perfect de-twinned crystals using synchrotron radiation in high resolution mode. This discrepancy is primarily caused by the fact the lowtemperature state is able to become "frozen", and the relaxation needs a few weeks at room temperature [23]. In particular, the results in [20] were obtained at heating of sample from a low

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temperature, so both Т1 and Т2 anomalies are almost invisible. Thus, for studying those features at Т1 and Т2 it is required to avoid cooling of the sample below the temperature of measurement.

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The composition of Y1-хСахBa2Cu3Oy allows varying the concentration of charge carriers when both oxygen content and the degree of Y/Ca substitution are changed [24]. Those two mechanisms are not completely independent [24-29]: the oxygen content can be described by the expression у = 7 − x / 2 , the addition of calcium leads to disorder in oxygen subsystem, buckling angle [Cu(2)-O(2)-Cu(2)] and planer distance [Cu(2)-O(2)] increase [25]. In [22], the temperature dependences of crystal structure during cooling of Y1-хСахBa2Cu3Oy, x=0; 0.1 with

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different oxygen content 6.6, 6.8, 6.95 are reported; the results were obtained by full profile analysis of powder X-ray diffraction data in the temperature range of 80-300 K. A decrease of the apical bond length because of a shift of apical oxygen atoms below ~225 K was found; the most noticeable shift of apical oxygen for compounds without calcium at y = 6.95 and for

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compounds with calcium at y=6.6 was observed. The question remains, what causes the shift of apical oxygen and the reduction of the apical bond length below ~225 K. To answer this

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question, we plan to examine the change of crystal structure during cooling from 300 to 80 K for Y1-xCaxBa2Cu3Oу varying both calcium and oxygen content and reveal appropriate changes in the electronic structure.

2. Materials and methods Samples were prepared using the conventional solid-state reaction method [22]. Pure Y2O3, BaCO3, CuO and CaO were weighed and then mixed according to the corresponding chemical formula of Y1-xCaxBa2Cu3Oy with x = 0, 0.05, 0.07, 0.1, 0.15, 0.2, respectively. Each mixture was ground thoroughly for 3 h and calcined at 930°C for 24 h in air. The resultant powders were reground, pressed into pellets and sintered at 950 °C for 24 h. Additionally, the 3

samples were heat-treated in air or oxygen (oxygen partial pressure lgP(O2) = −0.7 or 0,

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respectively) to obtain the desired oxygen content; it was determined using iodometry with an accuracy of ∆y= 0.025. Sample characteristics are shown in Table 1, unit cell parameters correspond to room temperature. X-ray diffraction structure analysis was performed with use of XRD-7000 Maxima (Shimadzu) diffractometer (Cu Kα radiation, graphite monochromator, external Si standard),

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scattering angle range 2θ = 10°−95° at room temperature and 20°−70° at lower temperatures. Low-temperature measurements at cooling were performed using Anton Paar TTK-450 chamber at vacuum 1·10-2 mbar, temperature was changed with a step of 7÷10 K, the error in maintaining of temperature was less than 0.1 K in the range of 160-300 K and less than 0.2 K in the range of 80-160 K.

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Crystal structure analysis was performed using GSAS package [30] starting from the following model of the crystal structure: Pmmm space group, a mixture Y/Ca (½ ½ ½), Ba (½½

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0.1843), Cu1 (0 0 0), Cu2 (0 0 0.3556), O1 (0 ½ 0), O2 (½ 0 0.3779), O3 (0 ½ 0.3790), apical oxygen O4 (0 0 0.1590) [31]. The scheme of the crystal structure is shown in Fig. 1. A shifted Chebyshev polynomial of the first type was used for background refinement. The profile function was accepted with 12 profile coefficients for Simpson's rule integration of pseudovoigt function,

the

temperature

factors

were

calculated

in

the

isotropic

form

U iso = exp( −8π e B sin 2 θ / λ2 ) [30]. The following discrepancy factors were achieved: weighted

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profile ωRp = 9-10%, the profile Rp = 7-8%, "goodness of the fit" χ2 = 1.6-1.8, Bragg factor RB = 6-7%. As an example, Fig. 2 shows the experimental, calculated and differential diffraction patterns for Y0.9Ca0.1Ba2Cu3O6.95 sample at 77 K. As well as in our previous works [22, 32], the

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calculations were performed in two stages. When the results for all the temperature points were obtained, some parameters (shift of zero, texture factor, asymmetry, Lorentz components of the

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line shape) were averaged over the whole temperature interval and fixed; using the analysis of initial results it was supposed that thermal parameters for all oxygen atoms may be treated as equal. Appliance of these suppositions at the second calculation stage leads to the same values of atoms coordinates and parameters of the Gaussian part of the shape of the lines as at the first stage, but to better statistic for all refined parameters. The temperature of superconducting transition was determined using magnetometry, the measurements were done with CFS-9T-CVT magnetometer (Cryogenic Ltd.) with a constant magnetic field (В = 0.05÷0.10 T) in FC- and ZFC-regimes. Calculations of the electronic structure were performed in a framework of the density functional theory using TB-LMTO-ASA package [33] based on the method of linearized muffintin orbitals in the atomic sphere approach. Integration in reciprocal space was performed using 4

the grid of 144 irreducible k-points with a full number of 10×10×6 = 600 k-points. An orbital

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basis included the muffin-tin orbitals corresponded to 5s-, 5p-, 4d- and 4f-states of Y; 6s-, 6p-, 5d- and 5f-states of Ba; 4s-, 4p- and 3d-states of Cu; and 3s-, 2p- and 3d-states of O. Atomic radii were ~3.5 a.u. for Y, 4.1 a.u. for Ba, 2.1 a.u. forСu1, 2.3 a.u. for Сu2, 2.0 a.u. for O and changed very slightly for unit cell parameters under consideration at different temperatures.

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3. Results and Discussions Comprising the oxygen content in the samples with different calcium quantity (Fig. 3a) one can notice that every set of data obtained at determined temperature and oxygen partial pressure may not be described by a universal expression у = 7 − x / 2 [27], but rather by

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у = y0 − x / 2 , where y0 is equilibrium oxygen content at used heat treatment condition. These lines with y0 = 6.9, 6.82 and 6.65 are plotted in Fig. 3a for the data obtained at 700°C in air,

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550°C in air and 500°C in oxygen, correspondingly. The experimental data deviate significantly from these lines, possibly due to a deviation from equilibrium at heat treatment and quenching. The temperatures of the superconducting transition are shown in the Fig. 3b. Evidently, for materials with high oxygen content the composition with x=0 is optimally doped, while for two other series of the samples with smaller oxygen content y~ 6.6 and y~ 6.8 the composition with x~0.7 is optimally doped.

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As one can see from the Table 1, (Y,Ca)-substitution at similar oxygen content leads to a very weak change of the unit cell dimensions; the parameters а and c increase while b decreases. Growth of oxygen content leads to a decrease of cell parameters а and с and to an increase of b,

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which matches with published data [25-26, 34-35]. The materials with either smallest (below 0.05) or highest (above 0.15) calcium content demonstrate very smooth temperature dependences of unit cell parameters, especially when oxygen content is as small as y~6.6÷6.8. In Fig. 4 unit

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cell dimensions a, b and c/3 as functions of temperature for two samples with calcium and oxygen content x=0, y=6.8 and x=0.1, y=6.64 are plotted. Paper format does not allow to plot temperature dependences of unit cell parameters for all studied samples so only these two compounds among all were chosen because of three reasons: first, samples with similar compositions were studied previously in details, bond lengths and atomic coordinates z(Ba) and z(O4) as function of temperature are published [22]; second, these materials represent “smooth” and “anomalous” cases for temperature dependence of function X(T), where X - a, b, c – unit cell parameters; third, they represent “small” and “large” change of the apical bond length on cooling (see Table 3 and text below). Atomic coordinates for these samples at present paper (Table 2) reproduce the data from ref. [22]. Character of function X(T) is quite similar to 5

published in [22]: for sample with x=0.1, y=6.60 the maximum of a(T) was observed at ~160 K

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[22], while for our sample with x=0.1, y=6.64 the maximum of a(T) was observed at ~170 K, other unit cell dimensions look rather smooth. Below ~150 K b and c parameters grow on cooling for sample with x=0, y=6.8 demonstrating negative thermal expansion. For a parameter this tendency is not evident. Some difference in absolute values of unit cell parameters in present paper and ref. [22] is caused by the following: in ref. [22] unit cell parameters were obtained as a result of full profile refinement; because of strong correlation with shift of zero, asymmetry and

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Lorentz components of the line shape, in general, such a way cannot provide correct values for unit cell parameters. So in Fig. 4 we plot unit cell dimensions obtained by the method of least squares for all measured diffraction lines in studied 2θ range.

A coefficients of the thermal expansion determined as functions of temperature

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α X = 1 / X ⋅ ( dX / dT ) , where Х - unit cell parameters, are shown in Fig. 5. The derivative was taken numerically using nearest 5-points smoothing. Except previously mentioned two samples

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x=0, y=6.8 and x=0.1, y=6.64 we also plot the data for intermediate composition x=0.07, y=6.68. The α X (T) functions have the N-shaped temperature dependence for the samples with x=0, y=6.8 (all dimensions) and x=0.07, y=6.68 (X = a, b). Since the dependences αХ(T) for the sample with x=0.07, y=6.68 look different from other samples, especially for X = c, the crystal structure parameters for this sample are also shown in Table 2. In whole, their values are

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between the respective values for the edge samples with x = 0 and x = 0.10, which makes it difficult to explain the behavior of thermal expansion for composition with x = 0.07. The temperature range between Т1 ~150 and Т2 ~225 K may be allocated; inside this range αХ(T) increases at cooling, this feature leads to N-shaped form of the αХ(T) curve. It is

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necessary to specify that T1 and T2 values may not be clearly evaluated from αХ(T) behavior. We assume mentioned above values Т1 ~150 and Т2 ~225 K as approximate data; much more

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accurate values can be obtained by measuring the absorption of ultrasound. For material without calcium and high oxygen content (Tc~93 K) two absorption peaks were observed using 50-90 kHz frequency: at 245 K (single peak) and 120-140 K (double peak) [18]. Among not only two shown samples but also among all investigated materials the compounds with calcium content x=0.1 regardless of the oxygen content possess low and relatively constant coefficient of thermal expansion along all crystallographic directions, that makes this material suitable for superconducting films and other composites. Earlier it was noticed [22] that the position of the apical oxygen atom O4 (Fig. 1) undergoes the most significant change as a function of temperature in comparison with the rest of the atoms in the structure. The change of the CuO5-pyramids height, that is the length of apical Cu2-O4 bond d Cu 2−O 4 = c ⋅ ( z (Cu 2) − z (O4)) , corresponding to cooling from 300 K to the 6

middle of the T1÷T2 interval was calculated as ∆d Cu 2−O 4 = d Cu 2−O 4 (300K ) − d Cu 2−O 4 (170K ) and ACCEPTED MANUSCRIPT shown in Table 3. One can see that for all samples this change is positive, it means the length of the apical bond is reduced at cooling down to 170 K; the strongest reduction is observed for the compounds with combinations of low oxygen (y~6.6) and small but not zero calcium content (0.05 < x < 0.10). To clarify the origin of the apical distance compression, the calculation of the electronic

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structure was performed for Y1-xCaxBa2Cu3Oy samples with two compositions mentioned above: x=0, y=6.8 and x=0.1; y=6.64 with small and large change of the apical distance at cooling (Table 3). The calculations were performed using atomic coordinates obtained at three temperatures 230 K, 170 and 110 K above, within and below the temperature interval Т1 ÷ T2

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(~150 ÷ ~225 K) (Table 3). The obtained results are shown in Figs. 6 - 8. Band structure near the Fermi energy is composed of bands with mainly Cu-3d and O-2p character and small admixture of Ba-5d and Y-4d states.

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The changes of electronic partial contributions of O4 and Ba are the most visible (Fig. 7). These changes include a shift of state-density peak (DOS) from Fermi level to lower energy at T~ 170 K, while for both higher and lower temperatures this peak is close to EF. It leads to a corresponding behavior for total DOS (Fig. 8). One can see that for Ba and O4 atoms at temperature T ~170 K, which is within Т1 ÷ T2 (~150 ÷ ~225 K) interval, a new peak of DOS at

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the energy ~0.4 eV below Fermi level arises (Fig. 7 b,d), it also results to a peak at total DOS (Fig. 8b). It reflects at the full electronic structure as appearance of the flat parts of the bands (Fig. 6 and inserts), which can be seen at the temperature of 170 K only (red line). For the sample without calcium doping and the oxygen content of y~6.8 the same features are observed,

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but much weaker; that is expectable as the changes in the crystal structure for this composition are also weaker. The appearance of the flat parts in the electronic structure may be interpreted as

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an increase of the charge carriers localization degree in the temperature interval T1÷T2 with participation of lattice deformation in a form of apical Cu-O bond compression. These changes of the band structure at cooling can be confirmed by results of K.B. Garg,

N.L. Saini and S. Dalela with co-authors [36-37], where a decrease of the itinerant hole density was observed at the same temperature ~150 K for underdoped de-twinned single crystals of YBCO [36] and both underdoped and overdoped La2-xSrxCuO4 single crystals [37] using polarized soft X-ray absorption spectroscopy. These phenomena were discussed in the context of holes aggregation. Another more recent result was obtained using XAS measurements at CuL3 edge for Bi2-yPbyCaCu2O8+δ single crystals where a minimum of itinerant hole density was observed at 180 K (near optimally doped state) and 220 K (underdoped state) [38]. A decrease of DOS near Fermi level accompanying a rise of DOS peak at ~0.4 eV below Fermi level was 7

observed in the present work (Figs. 7-8). Kinetic properties (conductivity and Hall constant) do

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not reveal any features which may be attributed to a minimum of charge carrier concentration or mobility [38]. Possibly, owing to the binding energy of the localized states is well below the Fermi level (~0.4 eV, Figs. 6-8) and the decrease of DOS at Fermi level is relatively small. The proof for connection of ultrasound absorption peak near 220 K to distortion of crystal structure was obtained in ref. [39] using Raman spectra. It is shown that compression of Ba-O layer when Sr was embedded in YBa2Cu3O7 leads to suppression of the peak. Authors [39]

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suggest that ultrasound absorption peak around 220 K is related to a dynamical crossover, a resolving process of the local polarons or bipolarons into the free carriers with increasing temperature. Disappearance of the DOS peak associated with the existence of flat sections of the

localized electronic state decays under heating.

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electronic structure at 230 K, as shown in present paper, is consistent with the assumption that

Therefore, the change of crystal structure of Y1-хСахBa2Cu3Oy with different content of

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calcium and oxygen has been studied. In accordance with published data [22] the main change is attributed to the apical Cu-O bond length reduced and corresponding increase of the Cu1-O4 bond between apical oxygen O4 and chain Cu-atom Cu1 below ~250 K appeared due to a shift of the apical oxygen atom. Below ~170 K a reciprocal change occurs. The calculation of the electronic structure shows rising of the peak of the electronic density of states for partial contributions of Ba and O4 atoms that can be interpreted as a charge carriers localization with

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participation of lattice deformation.

A sudden change of the crystal structure temperature dependence at (Y,Ca)-substitution may be an argument that charge carriers arising at an increase of oxygen content and calcium addition are different. The charge carriers appeared as a result of (Y,Са) substitution are more

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localized [40-42] and, consequently, have a stronger influence on atomic interactions than free charge carriers caused by oxygen. It is possible to suppose that some initial degree of charge

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carriers localization leads to an increase of the “brightness” of the crystal structure anomaly near ~150 K (compare a(T) data for compounds with and without calcium in Fig. 4).

Conclusion

Variation of crystal structure of Y1-хСахBa2Cu3Oy with different content of oxygen and

calcium has been studied. The main changes are related to the compression of the apical Cu-O bond length below ~250 K due to the shift of apical oxygen O4. Below ~170 K an initial undistorted state returns. Calculation of electronic structure shows the emergence of the peak of the density of states below ~0.4 eV Fermi level for Ba and O4 atoms accompanying reducing the apical bond 8

length, which is interpreted as the localization of charge carriers with participation of lattice

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deformation. At the same time with rising of DOS peak a decrease of density of states at Fermi level occurs. Composition with x = 0.1 and y = 6.8 shows low and relatively constant thermal expansion coefficient for all crystallographic directions. This makes this material attractive to use in superconducting composites.

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Acknowledgements

The work was performed in accordance of state project № 0396-2014-0001 of IMET UrD RAS, at partial financial support of UrD RAS, project № 15-17-2-16, use of the equipment of the

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Center “Ural-M”. A.V.L. thanks the “Dinasty” foundation for support.

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Figure captions ACCEPTED MANUSCRIPT Fig. 1. The scheme of the orthorhombic crystal structure for Y1-xCaxBa2Cu3Oy. Fig. 2. The experimental (red), calculated (green) and differential (magenta) diffraction patterns for Y0.9Ca0.1Ba2Cu3O6.95 sample at 77 K; line positions according to Pmmm space group

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are shown as black bars.

Fig. 3. Oxygen content (a) and superconducting transition temperature (b) as functions of calcium content for Y1-xCaxBa2Cu3Oy annealed at 700°C on air (black symbols), 550°C

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on air (grey symbols) and 500°C in oxygen (white symbols).

Fig. 4. Unit cell parameters a (red, left axis), b (black, right axis) and c/3 (blue, right axis) as

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functions of temperature for Y1-xCaxBa2Cu3Oy with calcium and oxygen content x=0, y=6.8 (open symbols) and x=0.1, y=6.64 (filled symbols). Fig. 5. Temperature dependences of the thermal expansion coefficient α X = 1 / X ⋅ ( dX / dT ) , where Х= a, b, c for Y1-xCaxBa2Cu3Oy samples with calcium and oxygen content x=0,

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y=6.64 (white), x=0.07, y=6.68 (red) and x=0.1, y=6.8 (black).

Fig. 6. Electronic structure of Y1-xCaxBa2Cu3Oy with x=0.1; y=6.6 at different temperatures 230 K (green), 170 K (red) and 110 K (black). Fermi level (EF) corresponds to zero at the

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energy scale. In the insets some details of the band structure near Y, between S and X, and near T point are shown, correspondingly.

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Fig. 7. Partial atomic contributions of O4 and Ba atoms in the electronic density of states for Y1-xCaxBa2Cu3Oy with x=0, y=6.8 (a, c) and x=0.1; y=6.6 (b, d) at different temperatures 230 K (green), 170 K (red) and 110 K (black).

Fig. 8. Total densities of states for Y1-xCaxBa2Cu3Oywith x=0, y=6.8 (a) and x=0.1; y=6.6 (b) at temperature 230 K (green), 170 K (red) and 110 K (black).

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ACCEPTED MANUSCRIPT Table 1. Characteristics of studied samples Y1-xCaxBa2Cu3Oy Unit cell dimensions, Å

Sample

Synthesis details

b

c

T, °C

lgP(O2)

YBa2Cu3O6.59

3.8452(5)

3.8781(5)

11.731(1)

700

-0.7

YBa2Cu3O6.8

3.8364(4)

3.8928(4)

11.725(1)

550

-0.7

YBa2Cu3O6.9

3.8292(4)

3.8907(4)

11.684(1)

500

0

Y0.95Ca0.05Ba2Cu3O6.58

3.8502(4)

3.8805(4)

11.730(1)

700

-0.7

Y0.95Ca0.05Ba2Cu3O6.78

3.829(4)

3.8906(4)

11.681(1)

550

-0.7

Y0.95Ca0.05Ba2Cu3O6.82

3.8388(4)

3.8998(4)

11.683(1)

500

0

Y0.93Ca0.07Ba2Cu3O6.68

3.8429(5)

3.8746(5)

11.728(1)

700

-0.7

Y0.93Ca0.07Ba2Cu3O6.8

3.8331(5)

3.893(5)

11.716(1)

550

-0.7

Y0.93Ca0.07Ba2Cu3O6.85

3.8294(4)

3.8954(4)

11.686(1)

500

0

Y0.9Ca0.1Ba2Cu3O6.64

3.8443(5)

3.8696(5)

11.735(1)

700

-0.7

Y0.9Ca0.1Ba2Cu3O6.79

3.8388(4)

3.8854(4)

11.690(1)

550

-0.7

Y0.9Ca0.1Ba2Cu3O6.88

3.8344(4)

3.8887(4)

11.697(1)

500

0

Y0.85Ca0.15Ba2Cu3O6.57

3.8571(3)

3.8764(3)

11.724(1)

700

-0.7

Y0.85Ca0.15Ba2Cu3O6.81

3.8313(4)

3.8840(4)

11.692(1)

550

-0.7

Y0.85Ca0.15Ba2Cu3O6.86

3.8387(4)

3.8869(4)

11.691(1)

500

0

Y0.8Ca0.2Ba2Cu3O6.58

3.8568(4)

3.8674(4)

11.691(1)

700

-0.7

Y0.8Ca0.2Ba2Cu3O6.73

3.8382(4)

3.877(4)

11.697(1)

550

-0.7

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a

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Table 2. Crystal structure parameters and “goodness of the fit” χ2 for Y1-xCaxBa2Cu3Oy samples x=0.10, y=6.64

170 K

110 K

230 K

170 K

110 K

230 K

170 K

110 K

3.835(1) 3.892(1) 11.707(2) 0.1871(1) 0.3557(5) 0.371(1) 0.372(1) 0.162(1) 0.004(4) 0.003(2) 0.027(3) 0.006(6) 0.006(3) 1.64

3.835(1) 3.888(1) 11.700(2) 0.1871(3) 0.3563(5) 0.374(1) 0.375(1) 0.164(1) 0.009(4) 0.006(2) 0.014(3) 0.003(6) 0.002(3) 1.68

3.834(1) 3.891(1) 11.697(2) 0.1865(3) 0.3572(5) 0.369(1) 0.379(1) 0.167(1) 0.003(4) 0.007(2) 0.010(3) 0.006(6) 0.007(3) 1.74

3.841(1) 3.876(1) 11.717(2) 0.1892(1) 0.3589(5) 0.376(1) 0.369(1) 0.155(1) 0.005(4) 0.001(2) 0.025(3) 0.001(6) 0.001(6) 1.92

3.840(1) 3.874(1) 11.701(2) 0.1898(2) 0.3583(4) 0.374(1) 0.367(1) 0.157(1) 0.003(4) 0.001(2) 0.027(3) 0.001(6) 0.001(6) 1.87

3.838(1) 3.874(1) 11.699(2) 0.1870(2) 0.3590(4) 0.374(1) 0.366(1) 0.156(1) 0.001(4) 0.002(2) 0.038(3) 0.001(6) 0.001(6) 1.86

3.844(1) 3.868(1) 11.723(2) 0.1916(1) 0.3599(5) 0.357(1) 0.367(1) 0.155(1) 0.005(4) 0.003(2) 0.025(3) 0.005(6) 0.021(6) 1.69

3.846(1) 3.867(1) 11.719(2) 0.1919(1) 0.3584(5) 0.341(1) 0.371(1) 0.184(1) 0.007(4) 0.002(2) 0.033(3) 0.008(6) 0.027(6) 1.66

3.841(1) 3.868(1) 11.714(2) 0.1917(1) 0.3583(4) 0.363(1) 0.367(1) 0.144(1) 0.010(4) 0.005(2) 0.020(3) 0.009(6) 0.030(6) 1.67

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a, Å b, Å c, Å z(Ba) z(Cu2) z(O2) z(O3) z(O4) Uiso(Y) Uiso(Ba) Uiso(Cu1) Uiso(Cu2) Uiso(O) χ2

x=0.07, y=6.68

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x=0, y=6.80

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ACCEPTED MANUSCRIPT Table 3. The change of the apical bond length (Å) on cooling from 300 K to 170 K Oxygen content

Ca content

6.6 0.18 0.21

0 0.05 0.07 0.1 0.15

6.7 0.01 0.05

0.04 0.01 0.02

6.87 0.09 0.07 0.09 0.05 0.10

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0.34 0.07

6.8 0.05

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o Crystal structure of Y1-хСахBa2Cu3Oy is studied in temperature range 80-300 K. o Shrink of the apical Cu-O bond in temperature interval ∆T = 170 - 225 K is found. o Electronic structure above, below and within ∆T interval is calculated. o Electronic states density peak is found within ∆T interval.

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o This peak is explained as a result of charge carriers localization.