Local structural studies of the cubic Cd_1–xCa_xO system through Cd K-edge extended X-ray absorption spectroscopic studies

Cd K-edge extended X-ray absorption fine-structure spectroscopic studies were carried out on Cd_1–xCa_xO (0 ≤ x ≤0.9) solid solutions and the first and second nearest neighbour (NN) distances and their mean square relative displacement σ² were estimated. The first NN distance, d_Cd–O(x), was found to be smaller than its expected value, a(x)/2, obtained from the X-ray diffraction measurements. It increases monotonically and non-linearly with a negative curvature, comparable with that of the a(x) value variation. The variation σ² of the 1NN with x is consistent with a disordered solid solution model. The 2NN distances d_Cd–Cd(x) and d_Cd–Ca(x) are found to follow the average values obtained by X-ray diffraction with d_Cd–Ca(x) > d_Cd–Cd(x). From detailed analysis it is argued that the solid solution exhibits a bimodal distribution of the 1NN distances, d_Cd–O(x) and d_Ca–O(x), and that the system belongs to a persistent type.

1. Introduction

The effect of local atomic rearrangements on the properties of semiconductor alloys has been emphasized recently on the basis of both experimental and theoretical results (Kuzmin et al., 1993; Tsai et al., 1990; Aldrich et al., 1994; Koteski et al., 2004). Such studies received further impetus with the availability of synchrotron radiation facilities to carry out extended X-ray absorption fine-structure (EXAFS) (Stern, 1974; Lytle et al., 1975; Stern et al., 1975; Bunker, 2010) and atomic pair distribution function (PDF) (Egami & Billinge, 2003) studies. These studies confirm that the local atomic arrangements show substantial deviation from the average structure, though the translational symmetry prevails over much larger length scales (Kuzmin et al., 1995; Lebedev et al., 2001). The effects of such deviation include the reduced point-group symmetry, lattice distortions and the modification in the bonding. These in turn affect the physical, optical and electronic properties, which are of fundamental importance for the semiconducting materials. Hence, detailed local structural studies have become of importance to understand not only the average structural changes on alloying but also optical and electronic properties.

Recently we reported a wide and tunable band gap II–VI semiconducting oxide Cd_1–xCa_xO system. Although the end members CdO and CaO crystallize in the NaCl structure with a lattice parameter mismatch of 2.5%, the lattice parameter (a) variation of their solid solution deviates from a linear variation (Vegard's law). The variation exhibits a negative curvature characterized by a bowing parameter b = 0.03088 (Srihari et al., 2011). Employing a Williamson–Hall type plot (Williamson & Hall, 1953) obtained from the profile analysis of powder X-ray diffraction (XRD), the strain as a function of Ca substitution was calculated. It is found that the strain is negligibly small, a generic feature of the oxides. This implies the absence of strain as a field over the coherently diffracting volume. This clearly indicates that substitution results in local structure deviation from the average cubic symmetry, warranting a detailed structural study at the atomic level. EXAFS spectroscopy is a widely used tool for such studies and is preferred over PDF studies since it selectively probes the local structure around an absorbing species. In this paper we report Cd K-edge EXAFS studies on the Cd_1–xCa_xO system. Our studies indicate that the first nearest neighbour (1NN) distance d_Cd–O(x) is smaller than a(x)/2 and exhibits a negative deviation with a bowing parameter same as that of a(x)/2. On the other hand, the second nearest neighbour (2NN) distance d_Cd–Cd/Ca(x) closely follows the variation of a(x)/2^1/2. It is shown that the 1NN distances d_Cd–O(x) and d_Ca–O(x) are different and the optical properties of this system would exhibit characteristics of a persistence mode system.

2. Experimental details

Solid solutions of the Cd_1–xCa_xO system with x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9 were prepared by solid state reaction described in an earlier work (Srihari et al., 2011). Cd K-edge EXAFS measurements on the Cd_1–xCa_xO system were carried out on beamline NW10A of the Photon Factory Advanced Ring (PF-AR), Tsukuba, Japan (Nomura, 2001; Nomura et al., 2007). The energy was varied from 500 eV below the Cd K-absorption edge, E₀ (= 26711 eV), to 1100 eV above it using a Si (311) double-crystal monochromator (d = 1.63747 Å). Measurements were carried out on finely ground powders of the Cd_1–xCa_xO system. The optimum sample quantity for each composition was estimated such that the effective thickness of the sample corresponds to approximately one absorption length for that composition. The required quantity of a diluting medium, boron nitride powder, was thoroughly mixed with the sample and pressed under moderate pressure into pellets of diameter 10 mm for measurement. Measurements under ambient conditions were carried out in transmission mode using ionization chambers for I₀ and I_t. Energy calibration was carried out using a standard Cd foil.

3. Data processing and analysis

The absorption spectra were processed using the Athena software package (Ravel & Newville, 2005) for edge alignment and pre-edge treatment (−500 to −150 eV before E₀) and background parameters were carefully chosen so that the data suffer the least from artefacts of such a processing. The post-edge treatment corresponding to removal of the K-edge absorption of free Cd²⁺ ions in the range 100–1100 eV above E₀ was carried out to obtain the EXAFS signal, χ(k). The χ(k) signal was then Fourier transformed (FT) in the range 2 Å⁻¹ ≤ k ≤ 14.5 Å⁻¹ to obtain the real, Re[χ(R)], imaginary, Im[χ(R)] and absolute magnitude, |χ(R)|, of χ(R). The obtained χ(R) was weighted by k³ and fitted to a structural model using Artemis software (Newville, 2001) to obtain local structural information; interatomic distances or the effective path length and mean square relative displacement, σ², with reference to the absorbing Cd²⁺ ion. In the analysis, single-scattering (SS) and multiple-scattering (MS) paths with up to four legs within a cluster size R_max = 6 Å were included. In the present study, the fitting was carried out in the range 2 Å ≤ R_eff ≤ 6 Å with the Hanning window and dR = 1 Å; R_eff is the effective path length. Although our primary interest is limited to the first and second NN, paths with higher R_eff need to be included since they form `leakage' paths into the low-R range.

The following strategy was adopted to incorporate atom substitution, which is not a standard feature of the Artemis package. It is assumed that the substitution is random, a fair assumption for the NaCl-type structure. Additional Ca²⁺ backscattering paths were included such that for every path (SS or/and MS paths) involving the Cd²⁺ ion a corresponding identical path involving the Ca²⁺ ion is also present. The degeneracy N (coordination number in the case of SS paths or multiplicity in the case of MS paths) of such paths was proportioned between Cd²⁺ and Ca²⁺ paths conforming to the nominal composition of the sample: N_Cd = Nx; N_Ca = N(1 − x). This methodology provides a scope to extract the EXAFS parameters of both the 2NN paths, Cd–Cd and Cd–Ca. In the case of random alloys it is known that the distribution of NN atoms around the associated average coordination shell is non-Gaussian (Crozier et al., 1988), especially so in the case of the Cd_1–xCa_xO system, wherein the atomic mass difference between Cd and Ca is large. Hence, the cumulant expansion method, which does not assume any specific form for the distribution, was adopted to fit the χ(R) signal (Bunker, 2010; Crozier et al., 1988). Accordingly, the first (C1), second (C2), third (C3) and fourth (C4) cumulants correspond to the NN distance or effective (scattering) path length R_eff, the mean square relative displacement (MSRD) σ² of the NN atoms with reference to the absorbing atom, skewness in the distribution and its weightage, respectively. While fitting, correlated parameters were consistently floated and it was seen that the number of floating parameters is always less than the number of independent parameters N_ind, as given by the Nyquist criteria (Stern, 1993).

Although we have considered 2–6 Å for obtaining structural parameters, the analysis was broken to overlapping windows in R-space. For a given fitting, only a few paths were included such that the total number of floated parameters obeys the Nyquist criteria. In all the fitting procedures the pairs of correlated parameters, S₀ and σ² and E₀ and ΔR, floated simultaneously. Subsequently, this window range (in R) was sifted so as to include at least one path of the previous fitting session and its parameters were fixed. The entire process was iterated so that good fitting and meaningful errors in the floated parameter were obtained.

4. Results and discussion

Fig. 1 shows the Cd K-edge absorption spectra of the Cd_1–xCa_xO solid solution. The spectra did not show perceptible changes in the XANES range (26611–26775 eV), except for a shift in the position of the first peak to higher energies with x. This shift implies changes in bond lengths and in the nature of bonding, corroborating our earlier inference deduced from the electron density distribution from the powder XRD data (Srihari et al., 2011). On the other hand, substantial and systematic changes are observed in the EXAFS, the k³-weighted signal χ(k) (Fig. 2), especially in the range 4 Å⁻¹ ≤ k ≤ 12.5 Å⁻¹, indicating changes in the environment of Cd with Ca substitution. In passing, it is remarked that the absence of noise in k³χ(k), even up to 16 Å⁻¹, indicates that the samples are free of pin holes. The absolute magnitude of χ(R) of the solid solution in the range 1–6 Å is shown in Fig. 3. It exhibits several peaks and substructures: a peak at ∼2.2 Å corresponding to the 1NN distance, viz. d_Cd–O ≃ a/2, estimated from powder XRD studies (Srihari et al., 2011). The shift in the position of this peak to higher R values with x indicates an increase of d_Cd–O(x) with x, conforming to the lattice dilation with Ca substitution. The second peak is centered at about ∼3.37 Å and exhibits a shoulder at R ≃ 3.05 Å; the shoulder to the second peak in CdO may be due to non-linearity in backscattered photoelectron intensity from the Cd atom. Although the position of the second peak corresponds approximately to the 2NN distance, viz. d_Cd–Cd/Ca ≃ a/2^1/2, as will be seen below, the paths with still higher effective scattering length also contribute to this peak. Figs. 4 and 5 show a structural model fit to the real part, imaginary part and absolute magnitude of χ(R), respectively, for x = 0.3. Contributions from a few representative scattering paths up to 1–3NN are shown. It is seen from these figures that the 1NN signal is not contaminated much by higher-order paths. The contribution of the 2NN, Cd–Cd and Cd–Ca paths are not in phase [Figs. 4(a) and 4(b)] and their contributions are located at about 3.37 Å and 3.05 Å, respectively, in |χ(R)|. The contribution of the Cd–Ca path to the second peak also falls near to the shoulder of the Cd–Cd contribution. From Fig. 5 it is observed that the Cd–Ca contribution to the second peak is at a lower R value compared with that of the Cd–Cd contribution; this may be due to the difference in the phase shift between the scattered electron wave from the Cd and Ca atoms. Also, it can be seen from Fig. 5 that contributions from the 3NN and higher-order paths strongly overlap with those of the 2NN contributions; e.g. contributions from the MS path, Cd(absorbing)–Cd(at face-centered positions)–O1(at edge-centered positions), and SS path, Cd–O2 (body-centered position). This necessitates one to include the paths with R_eff = 6 Å during fitting. From the analysis of |χ(R)|, the NN distances d_Cd–O(x), d_Cd–Cd(x) and d_Cd–Cd(x) were estimated and are tabulated in Table 1 along with σ², S₀², E₀ and the R-factor, a measure of the goodness of fit.

Table 1
Estimated first (d_Cd–O) and second (d_Cd–Cd and d_Cd–Ca) distances, mean square relative displacements σ², third cumulant C3, a measure of the skewness in the distribution of the nearest neighbour distances, S₀², E₀ from the theoretical fit to χ(R), along with the R-factor, a measure of the goodness of fit

X	R (Å)	σ^2 (Å^2)	C3 (10^−4)	S0^2	E0	R-factor
1NN Cd–O
0	2.347 (7)	0.0027 (8)	1.71 (1)	0.796 (5)	4.1 (2.6)	0.00179
0.1	2.350 (8)	0.003 (2)	2.36 (4)	0.730 (9)	5.8 (1.2)	0.00001
0.2	2.355 (3)	0.0028 (8)	2.24 (2)	0.684 (6)	5.5 (0.7)	0.00105
0.3	2.357 (6)	0.0039 (8)	2.5 (3)	0.739 (9)	4.5 (0.9)	0.00003
0.4	2.354 (2)	0.004 (1)	2.44 (1)	0.740 (6)	5.8 (0.4)	0.0023
0.5	2.364 (1)	0.0015 (1)	2.5 (3)	0.598 (5)	5.3 (0.5)	0.0008
0.6	2.368 (3)	0.004 (1)	2.18 (6)	0.713 (2)	5.2 (0.6)	0.00133
0.7	2.374 (3)	0.003 (1)	2.65 (2)	0.742 (3)	5.0 (0.5)	0.0013
0.8	2.380 (5)	0.0027 (2)	3.71 (1)	0.695 (2)	4.7 (0.6)	0.00001
0.9	2.386 (4)	0.0021 (1)	3.36 (4)	0.675 (5)	4.8 (0.6)	0.00219
2NN Cd–Cd
0	3.329 (6)	0.0088 (6)	0.13 (2)	1.087 (9)	2.7 (0.1)	0.00179
0.1	3.324 (8)	0.0081 (8)	0.2 (1)	1.018 (5)	3.4 (0.1)	0.00001
0.2	3.332 (7)	0.0081 (7)	4 (2)	1.002 (9)	3.8 (0.1)	0.00105
0.3	3.340 (1)	0.0079 (8)	6.2 (1)	1.035 (2)	3.4 (0.2)	0.00003
0.4	3.348 (9)	0.0079 (5)	4.9 (6)	0.945 (3)	4.0 (0.2)	0.0023
0.5	3.363 (5)	0.0081 (5)	6.1 (1)	0.849 (7)	3.4 (0.5)	0.0008
0.6	3.362 (2)	0.0072 (4)	3.7 (3)	0.868 (2)	4.5(0.9)	0.00133
0.7	3.372 (1)	0.0069 (3)	1.4 (2)	0.829 (9)	3.0 (0.2)	0.0013
0.8	3.380 (1)	0.0069 (7)	1.6 (4)	0.716 (1)	3.7 (0.2)	0.00001
0.9	3.389 (4)	0.0047 (1)	1.53 (1)	1.003 (6)	5.5 (0.9)	0.00219
2NN Cd–Ca
0	–	–	–	–	–	0.00179
0.1	3.336 (1)	0.0105 (2)	0.92 (1)	1.465 (6)	1.1 (1.0)	0.00001
0.2	3.345 (8)	0.011 (1)	1.35 (3)	1.055 (9)	1.5 (0.7)	0.00105
0.3	3.354 (7)	0.0115 (9)	1.87 (5)	0.998 (2)	1.6 (0.4)	0.00003
0.4	3.361 (2)	0.012 (1)	3.69 (7)	1.096 (6)	2.3 (0.3)	0.0023
0.5	3.378 (9)	0.0107 (4)	5.52 (1)	1.283 (2)	3.6 (0.3)	0.0008
0.6	3.378 (2)	0.0118 (6)	3.73 (2)	1.371 (1)	3.2 (0.3)	0.00133
0.7	3.388 (1)	0.0111 (5)	3.69 (5)	1.463 (7)	3.7 (0.1)	0.0013
0.8	3.397 (8)	0.0112 (1)	3.68 (2)	1.409 (5)	3.5 (0.1)	0.00001
0.9	3.397 (8)	0.0117 (3)	2.66 (4)	1.475 (4)	4.2 (0.1)	0.00219

Figure 1 Measured Cd K-edge X-ray absorption spectra [μ(E)] as a function of incident photon energy for Cd1–xCaxO (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9). The curves are vertically displaced for clarity.

Figure 2 Extracted EXAFS [k^3χ(k)] as a function of k, for Cd1–xCaxO (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9). The curves are vertically displaced for clarity and the peaks having major changes with x are marked with arrows.

Figure 3 Absolute magnitude of the χ(R) signal as a function of radial distance from Cd for Cd1–xCaxO (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9). The curves are vertically displaced for clarity.

Figure 4 Theoretical fitting of the real part (a) and imaginary part (b) of the χ(R) signal, for the x = 0.3 sample. Contributions from Cd–O, Cd–Cd and Cd–Ca are plotted. The fitting range is 2–4 Å with the Hanning window.

Figure 5 Theoretical fitting of the absolute magnitude of χ(R) for x = 0.3. Contribution from paths 1 to 4 are indicated in the first two shells for the x = 0.3 sample (fitting range 2–4 Å) with the Hanning window. Contributions from higher paths are also plotted to indicate leakage of higher paths into lower paths.

In Fig. 6 the variation of d_Cd–O(x) (red circles) is compared with the corresponding distance a(x)/2 (black squares). It is seen that d_Cd–O(x) monotonically increases and exhibits a negative deviation from a linear behavior. The variation of d_Cd–O(x) was fitted to a second-order polynomial equation (Srihari et al., 2011) (red line, `Quadratic Fit') and the bowing parameter was estimated to be d_EXAFS = 0.026 Å. This value compares well with the bowing parameter for the lattice parameter variation, d_XRD = 0.03088 Å (Srihari et al., 2011). In contrast, the 2NN distances, d_Cd–Cd(x) and d_Cd–Ca(x), increase almost linearly with x (Fig. 7) and are comparable with a(x)/2^1/2. It is also seen that d_Cd–Cd(x) is smaller than d_Cd–Ca(x) for all values of x, consistent with the larger ionic radius of Ca²⁺ compared with that of Cd²⁺. Variation of , a measure of both thermal and structural disorder, is shown in Fig. 8. The 1NN of undoped CdO itself has a finite value. For x = 0.5, the value of is found to be smaller even than that for the end members. Barring this, the overall variation of exhibits a more or less symmetric variation with a positive curvature having a maximum value of ∼0.0043 Ų for x ≃ 0.5 (Fig. 8). On the other hand, the disorder associated with the 2NN is comparable, ≃ ≃ 0.01 Å. This is much higher compared with that of 1NN and did not exhibit any systematic variation with Ca substitution. The third cumulant of the 1NN distribution for the CdO (x = 0) is ∼2 × 10⁻⁴ Å, implying a small skewness. It increases with armchair structure to ∼3 × 10⁻⁴ Å for x = 0.9. While the variation in the skewness in the distribution of d_Cd–Ca(x) was found to be negligible with no systematic variation, the variation in the skewness in the distribution of d_Cd–Ca(x) exhibits a bell shape: it is either zero or negligibly small for x = 0, 0.1, 0.8 and 0.9 and has a maximum value of ∼6 × 10⁻⁵ ų for x = 0.5. Thus, the skewness in the 2NN distribution is estimated to be one order less compared with that of 1NN.

Figure 6 Variation of the first nearest neighbor distance dCd–O(x) with x and its fit to a second-order polynomial function. Variation of a(x)/2 with x (Srihari et al., 2011) and its fit to a second-order polynomial function and expected variation of dCd–O(x) with x.

Figure 7 Variation of the second nearest neighbour distances dCd–Cd(x) and dCd–Ca(x) with x. The variation of a(x)/2^1/2 (Srihari et al., 2011) is also given for comparison.

Figure 8 Variation of the mean square relative displacement σ^2 for the first nearest neighbor dCd–O(x). The solid line is a guide for the eye.

In the past, local structural studies through PDF/EXAFS have been carried out on semiconducting alloys, which can be broadly divided into two groups. For the alloys belonging to the first group, the variation of the lattice parameter follows Vegard's law: Ga_1–xIn_xAs (Egami & Billinge, 2003), Hg_1–xCd_xTe (Pong et al., 1989), Rb_1–xK_xBr (Boyce & Mikkelsen, 1985), Th_1–xU_xO₂ and Th_1–xPu_xO₂ (Hubert et al., 2006). For the alloys belonging to the second group, the variation of the lattice parameter deviates from Vegard's law: Mg_1–xNi_xO (Kuzmin et al., 1995), Mg_1–xCo_xO (Kuzmin et al., 1993) and Mg_1–xFe_xO (Waychunas et al., 1994). In the first group of alloys the 1NN cation–anion distances (e.g. d_Ga–As and d_In–As for Ga_1–xIn_xAs and d_Th–O and d_Pu–O for Th_1–xPu_xO₂) are either almost the same or are comparable with those of the respective end-member cation–anion distances or they vary linearly with a substantially small slope comparable with the lattice parameter variation. Among the members belonging to the second group, Kuzmin & Mironova (1998) have carried out detailed local structural investigations on Mg_1–xNi_xO solid solution (space group ) employing Mg- and Ni-edge EXAFS. Although the lattice parameter of this system exhibits a negative deviation from Vegard's law, with b_XRD ≃ 0.01 Å (Kuzmin & Mironova, 1998), both the 1NN distances d_Mg–O(x) and d_Ni–O(x) are reported to vary linearly. These authors have used the decrease of 2NN distances d_Mg–Mg(x) and d_Ni–Ni(x) to explain the lattice parameter variation. This is to be contrasted with the Cd_1–xCa_xO system under investigation. For the Cd_1–xCa_xO system the 1NN distance d_Cd–O(x) exhibits a negative deviation with a bowing parameter comparable with that of the lattice parameter variation. Considering that d_Cd–O(x) is always smaller than a(x)/2, Cd_1–xCa_xO mimics the behavior of the former group of alloys with the exception that both the lattice parameter and the d_Cd–O(x) variation deviates from Vegard's law. Compared with semiconducting alloys with wurzite or chalcopyrate structure, the difference between a(x)/2 and d_Cd–O(x) for the Cd_1–xCa_xO system is smaller, as has been observed for purely ionic systems like K_1–xRb_xBr (Boyce & Mikkelsen, 1985). Since the wurzite structure allows larger changes in the bond angles rather than in the bond lengths, to minimize the strain, the end-members' 1NN distances are more or less preserved, leading to a larger difference between a(x)/2 and 1NN distances. On the other hand, the NaCl-type structure with the space group, to which the Cd_1–xCa_xO system belongs, does not allow for large bond angle variations and results in larger changes in the bond lengths. This leads to a smaller difference between a(x)/2 and d_Cd–O(x) compared with that of the systems belonging to the wurzite structure. Additionally, for systems where the lattice parameter variation obeys Vegard's law, the Phillips ionicity (Phillips, 1970, 1973) of the end members are closely matched (e.g. 0.31 for GaAs and 0.357 for InAs) and changes in the nature of the chemical bonding are not expected. The Phillips ionicity of CdO and CaO are widely different: 0.785 for CdO and 0.913 for CaO. Alloying of such end members brings out change in the nature of the chemical bonding; from more covalent-like for smaller values of Ca substitution to ionic bonding for higher Ca substitution (Srihari et al., 2011). This, coupled with the rather rigid cubic symmetry, is possibly responsible for the observed bowing in the variation of d_Cd–O(x) and hence in a(x).

To quantitatively explain the variation of d_Cd–O(x), it is necessary to consider the neighborhood of Ca. Owing to the proximity of the Cd L_I-edge (4018 eV) to the Ca K-edge (4038.5 eV), Ca L-edge EXAFS could not be carried out and d_Ca–O(x) could not be estimated. However, the local structure averaged over a much larger length scale should conform to the crystal structure as obtained from the diffraction studies. In other words, the sum of the compositional weighted 1NN distances of Cd and Ca should give the lattice parameter as (Azoulay et al., 1982)

In the above equation, the fitted values of a(x) and d_Cd–O(x) were used to estimate d_Cd–O(x), and its variation is plotted as the solid blue line (`expected d_Cd–O') in Fig. 6. Thus the EXAFS studies indicate a bimodal distribution for 1NN distances.

As seen from Fig. 7, the variation of the 2NN distance d_Cd–Cd/Ca(x) closely follows that of the average 2NN distance a(x)/2, in contrast to the 1NN distance d_Cd–O(x) variation. Such a behavior has been reported for both purely covalent and ionic systems like the Ga_1–xIn_xAs (Mikkelsen & Boyce, 1983) and K_1–xRb_xBr (Boyce & Mikkelsen, 1985) systems, respectively. This is explained by Boyce & Mikkelsen (1985) by invoking a radial force model, wherein bond-stretching radial forces alone are considered while weak bond-bending forces are omitted (Shih et al., 1985). Alternatively, one should also take into account (x) to explain this (Frenkel et al., 1993). Upon lattice dilation, d_Cd–Cd/Ca(x), as expected, increases by 0.06 Å, from 3.3292 Å (x = 0) to 3.3892 Å (x = 0.9). However, no appreciable changes in (x) are observed indicating that the `packing' around 2NN Cd/Ca is preserved over the entire composition range, as has been reported for the K<sub>1–x</sub>Rb<sub>x</sub>Br system (Frenkel et al., 1993). Hence, an increase in d<sub>Cd–Cd/Ca</sub>(x) is interpreted as an increase in the size of the ionic species with x. With this in the background, variations in (x) clearly indicate that `packing' around 1NN O is rather loose and it undergoes substantial local structural deviation to accommodate the substitution of the larger Ca²⁺ for the smaller Cd²⁺.

5. Conclusions

Cd K-edge EXAFS measurements at room temperature were carried out on Cd_1–xCa_xO (0 ≤ x ≤ 0.9). Employing the cumulant expansion method, the first [d_Cd–O(x)] and second nearest neighbour distances [d_Cd–Cd(x) and d_Cd–Ca(x)] and mean square relative displacement, σ², were estimated. It was found that the d_Cd–O(x) variation exhibits a negative deviation from linearity with a curvature quite close to that of the lattice parameter variation a(x) and is smaller than a(x)/2. By analyzing the variation of d_Cd–O(x) and a(x), it is inferred that the 1NN distance for Ca, d_Ca–O(x), is larger than d_Cd–O(x), implying a bimodal distribution for the nearest neighbour distances. From this, it is expected that the optical properties of the Cd_1–xCa_xO system should conform to a persistent type system. From the linear increase in 2NN distances d_Cd–Cd(x) and d_Cd–Ca(x), with the associated σ² being almost constant, it is reasoned that the ionic sizes of the species are concentration-dependent.