Electronic structure of ZrCuSiAs and ZrCuSiP by X-ray photoelectron and absorption spectroscopy

Article abstract of DOI:10.1016/j.jssc.2010.04.032

The electronic structures of quaternary pnictides ZrCuSiPn (Pn=P, As) were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES). Shifts in the core-line XPS and the XANES spectra indicate that the Zr and C

Full text of DOI:10.1016/j.jssc.2010.04.032

ARTICLE IN PRESS
Journal of Solid State Chemistry 183 (2010) 1536–1544
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Electronic structure of ZrCuSiAs and ZrCuSiP by X-ray photoelectron and
absorption spectroscopy
Ã
Peter E.R. Blanchard, Ronald G. Cavell, Arthur Mar
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
a r t i c l e i n f o
a b s t r a c t
Article history:
The electronic structures of quaternary pnictides ZrCuSiPn (Pn¼P, As) were analyzed by X-ray
photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES). Shifts in the
core-line XPS and the XANES spectra indicate that the Zr and Cu atoms are cationic, whereas the Si and
Pn atoms are anionic, consistent with expectations from simple bonding models. The Cu 2p XPS and Cu
L-edge XANES spectra support the presence of Cu1+. The small magnitudes of the energy shifts in the
XPS spectra suggest significant covalent character in the Zr–Si, Zr–Pn, and Cu–Pn bonds. On progressing
from ZrCuSiP to ZrCuSiAs, the Si atoms remain largely unaffected, as indicated by the absence of shifts
in the Si 2p3/2 binding energy and the Si L-edge absorption energy, while the charge transfer from metal
to Pn atoms becomes less pronounced, as indicated by shifts in the Cu K-edge and Zr K, L-edge
absorption energies. The transition from two-dimensional character in LaNiAsO to three-dimensional
character in ZrCuSiAs proceeds through the development of Si–Si bonds within the [ZrSi] layer and
Zr–As bonds between the [ZrSi] and [CuAs] layers.
Received 22 February 2010
Received in revised form
21 April 2010
Accepted 24 April 2010
Available online 14 May 2010
Keywords:
XPS
XANES
Quaternary pnictides
& 2010 Elsevier Inc. All rights reserved.
1. Introduction
[RE3+O2ꢀ][M2+Pn3ꢀ] and [Zr4+Si2ꢀ][Cu1+As3ꢀ], deduced upon the
assumption of full electron transfer, seem to provide
a tidy
explanation for the origin behind this distortion. Apart from the
transition-metal atoms which can support partially filled d-states, all
atoms in REMPnO attain filled-shell electron configurations; thus,
REMPnO (and the other oxides) are classified as ‘‘normal valence’’
compounds. On the other hand, the Si atoms in ZrSiCuAs are deficient
with respect to the 8–n rule and must then form Si–Si bonds; that
is, ZrCuSiAs should be considered a polyanionic compound. The
formation of these Si–Si bonds contracts the structure within the ab
plane, concomitant with an expansion along the c-direction.
In view of these profound differences, it is worthwhile to re-
examine the electronic structure and bonding in ZrCuSiAs, as well
as its phosphide analogue ZrCuSiP. We take primarily an
experimental approach, applying X-ray photoelectron spectro-
scopy (XPS) and X-ray absorption near-edge spectroscopy
(XANES) to probe energy differences in core-level atomic states,
supplemented by band structure calculations to examine the
bonding character in these compounds. The results are compared
with LaNiAsO, a representative member studied in our previous
investigations of rare-earth arsenide oxides [9].
The tetragonal structure of ZrCuSiAs (space group P4/nmm (no.
129)) serves as the prototype for numerous compounds, with over
150 representatives [1]. By far the two largest subsets are the rare-
earth transition-metal pnictide oxides REMPnO [2], which are super-
conductors at relatively high temperatures when doped with fluorine
[3–5], and the rare-earth copper chalcogenide oxides RECuChO, which
are promising transparent p-type semiconductors [6]. It has been
fashionable to portray the structure of the pnictide oxides in terms
of a stacking of alternating [REO] and [MPn] layers, with both O and M
atoms in tetrahedral coordination geometries (Fig. 1a). In contrast,
there are only three examples of quaternary silicon pnictides known
so far: ZrCuSiAs itself, HfCuSiAs, and ZrCuSiP [7,8]. Ostensibly the
structure of ZrCuSiAs can also be described in terms of layers, [ZrSi]
and [CuAs], with Si and Cu atoms in the tetrahedral sites (Fig. 1b). The
c/a ratio is 2.0–2.3 for all the oxide representatives of this structure
type, but 2.6 for the three quaternary silicon pnictides [1].
Paradoxically, this expanded c/a ratio does not mean that the layers
are further apart in ZrCuSiAs! The reasons seem evident even from
just casual inspection of the figures. Within individual layers, the
tetrahedra are highly compressed along the c-direction in REMPnO,
but highly elongated in ZrCuSiAs. This distortion causes the Zr
atoms in the [ZrSi] layer to approach the As atoms in the [CuAs]
2. Experimental
˚
layer quite closely (2.8 A) [7]. The electron-precise formulations
2.1. Synthesis
Ã
ZrCuSiAs and ZrCuSiP were prepared according to literature
procedures [7,8]. Reagents were Zr wire (99.9%, A.D. Mackay), Cu
Corresponding author. Fax: +1 780 492 8231.
E-mail address: arthur.mar@ualberta.ca (A. Mar).
0022-4596/$ - see front matter & 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2010.04.032

ARTICLE IN PRESS
P.E.R. Blanchard et al. / Journal of Solid State Chemistry 183 (2010) 1536–1544
1537
Pn
Pn
Cu
Zr
M
RE
c
O
Si
c
b
b
Fig. 1. Crystal structure of (a) REMPnO showing non-interacting [REO] and [MPn] layers and (b) ZrCuSiPn (Pn¼P, As) showing interacting [ZrSi] and [CuPn] layers, stacked
along the c-direction.
powder (99.9999%, Cerac), Si pieces (99.99999%, Alfa-Aesar), As
powder (99.5%, Cerac), and red P powder (99.995%, Cerac). For
ZrCuSiAs, a stoichiometric mixture of ‘‘ZrSi’’ (previously prepared
by arc-melting), Cu, and As powders was placed within an
evacuated sealed fused-silica tube, which was heated to 873 K
over 1 day, kept at that temperature for 2 days, heated to 1173 K
over 1 day, kept at that temperature for 2 days, and water-
quenched. The product was reground and reheated at 1073 K for
4 days; this procedure was repeated three times. For ZrCuSiP, a
stoichiometric mixture of ‘‘ZrCuSi’’ (previously prepared by arc-
melting) and P was placed within an evacuated sealed fused-silica
tube, which was heated to 873 K over 1 day, kept at that
temperature for 1 day, heated to 1123 K over 1 day, kept at that
temperature for 2 days, and water-quenched. The product was
reground and reheated at 1123 K for 4 days; this procedure was
repeated three times. The products were judged to be single-
phase from their powder X-ray diffraction patterns collected on
an Inel powder diffractometer (Fig. S1 in Supplementary Data).
were sputter-cleaned with an Ar+ ion beam (4 kV, 10 mA) until
core-line peaks associated with surface oxides were no longer
observed in the XPS spectra. A small shoulder in the As 3d XPS
spectra indicated that a slight reduction of As occurred in
ZrCuSiAs; however, the binding energies (BE) in these and other
spectra were, within standard uncertainties, the same before and
after the sputtering procedure.
Survey spectra, collected with a BE range of 0–1100 eV, a pass
energy of 160 eV, a step size of 0.7 eV, and a sweep time of 180 s,
confirmed the expected chemical compositions (Zr:Cu:Si:
Pn¼1:1:1:1) for the samples examined. High-resolution core-line
spectra were collected with an energy envelope of appropriate
range (50 eV for Zr 3d; 60 eV for Cu 2p; 20 eV for Si 2p, As 3d, and P
2p), a pass energy of 20 eV, a step size of 0.05 eV, and a sweep time
of 180 s. No charge correction was required because the samples
are good conductors. The spectra were calibrated to the C 1s line
at 284.8 eV arising from adventitious carbon. As a check, another
suitable calibration technique is to set the first derivative of the
edge onset of the valence band, which should be at 0 eV for a
metallic sample [10]. The spectra were analyzed with use of the
CasaXPS software package [11]. The background arising from
energy loss was removed by applying a Shirley-type function and
the peaks were fitted to pseudo-Voigt (70% Gaussian and 30%
Lorentzian) line profiles to take into account spectrometer and
lifetime broadening effects. Table 1 lists average BE values
expressed to two decimal places, with uncertainties estimated
at better than 70.10 eV, as established by collecting multiple
spectra (Table S1 in Supplementary Data).
˚
˚
The unit cell parameters are a¼3.6608(8) A and c¼9.604(2) A for
˚
˚
ZrCuSiAs, and a¼3.5663(8) A and c¼9.421(2) A for ZrCuSiP, which
˚
are comparable to previously reported values (a¼3.6736(2) A and
˚
˚
c¼9.5712(9) A for ZrCuSiAs [7]; a¼3.5671(1) A and c¼9.4460(4)
˚
A for ZrCuSiP [8]).
2.2. XPS analysis
XPS spectra for ZrCuSiAs and ZrCuSiP were measured on a
Kratos AXIS 165 spectrometer equipped with a monochromatic
Al K
a
X-ray source (14 mA, 15 kV) and a hybrid lens with a spot
m2. The samples are air-stable but to minimize
2.3. XANES analysis
size of 700 ꢁ 400
m
exposure to air, they were handled in an Ar-filled glove box where
they were finely ground, pressed into In foil, mounted on a Cu
sample holder, and placed in a sealed container for transfer to the
analysis chamber of the spectrometer. The pressure inside the XPS
instrument was maintained between 10–7 and 10–9 Pa. Samples
X-ray absorption spectra were measured at the Canadian Light
Source (CLS) in Saskatoon, Saskatchewan. The bending magnet
soft X-ray microcharacterization beamline (SXRMB, 06B1-1) was
used for Zr L-edge spectra, the high-resolution spherical grating

ARTICLE IN PRESS
1538
P.E.R. Blanchard et al. / Journal of Solid State Chemistry 183 (2010) 1536–1544
Table 1
within the first Brillouin zone. For comparison, the band structure
Core-line BEs (eV) for ZrCuSiAs and ZrCuSiP.
of LaNiAsO was also calculated in a similar manner.
ZrCuSiAs
ZrCuSiP
a
3. Results and discussion
Zr 3d5/2
179.64(2)
933.68(3)
99.41(4)
41.13(6)
179.66(5)
933.70(6)
99.40(5)
a
Cu 2p3/2
Si 2p1/2
To parallel the approach taken previously to analyze the
electronic structure of the rare-earth arsenide oxides REMAsO,
we first break down the structure of ZrCuSiPn (Pn¼P, As) into its
nominally separate ‘‘layers’’, [ZrSi] and [CuPn]. Then we examine
the question of whether the layers interact by probing to what
extent charge transfer occurs within and between these layers.
As 3d5/2 or P 2p3/2
128.82(6)
a
For comparison, the Zr 3d5/2 BE is 178.91(3) eV in Zr metal and the Cu 2p3/2
BE is 932.86(4) eV in Cu metal.
monochromator beamline (SGM, 11ID-1) for Cu L-edge spectra,
and the variable line spacing plane grating monochromator
beamline (VLS PGM, 11ID-2) for Si L-edge spectra. The SXRMB
beamline is equipped with an InSb (111) double crystal mono-
chromator which provided a photon flux of ꢂ1011 photons/s, with
a resolution of 3.3 eV at 10 keV and a beamsize of approximately
3.1. [ZrSi] layer
The Zr 3d XPS spectra for ZrCuSiAs and ZrCuSiP were fitted to
two peaks corresponding to the spin–orbit-split 3d5/2 and 3d3/2
final states, in an intensity ratio of 3:2 (Fig. 2). (The weak shoulder
found at high BE in all spectra is interpreted as a surface oxide.)
The Zr 3d5/2 BEs in ZrCuSiAs (179.64(2) eV) and ZrCuSiP
(179.66(5) eV) are higher than in Zr metal (178.91(3) eV) [15],
indicating the presence of cationic Zr atoms, but they are much
lower than in ZrO2 (182.7(6) eV) [15], indicating that the charge is
nowhere as extreme as 4+ which would be expected if full
electron transfer were to occur. Indeed, a signature for electronic
delocalization is already evident in the asymmetric line shapes of
these peaks, skewed towards higher BE. Common in the spectra of
many elemental transition metals, including Zr, this asymmetry
arises when valence electrons, interacting with the core–hole
formed after photoionization, are excited into the continuum of
300 ꢁ 300
m
m2. Finely ground samples were mounted on carbon
tape and inserted into the vacuum chamber via a load lock.
Multiple scans of the Cu and Si L-edge XANES spectra were
collected for each sample, giving an estimated accuracy of
70.1 eV for the absorption energies. Spectra were collected in
total electron yield (TEY) and X-ray fluoresence yield (FLY) modes,
with a step size of 0.1 eV through the edge over various energy
ranges (Zr L-edge, ꢂ50 eV below to ꢂ500 eV above the edge;
Cu L-edge, ꢂ20 eV below to ꢂ65 eV above; Si L-edge, ꢂ30 eV
below to ꢂ30 eV above). The Zr spectra were calibrated against a
standard of FePO4, with the maximum of the first derivative of the
P K-edge set to 2153.0 eV [12]; the Cu and Si spectra were
calibrated against standards of the elemental metals, with the
maxima in the first derivatives of the L3-edges set to 932.7 eV for
Cu and 99.2 eV for Si [13]. Uncertainties of 70.1 eV for the
absorption edge energies were estimated by collecting multiple
scans for each sample.
The Pacific Northwest Consortium/X-ray Operations and
Research Collaborative Access Team (PNC/XOR-CAT), Sector 20
bending magnet beamline (20-BM), at the Advanced Photon
Source (APS) at Argonne National Laboratory was used to measure
Zr and Cu K-edge XANES spectra. The silicon (111) double crystal
monochromator on this beamline provided a photon flux of
ꢂ1011 photons/s with a resolution of 1.4 eV at 10 keV and a beam
size of approximately 1 ꢁ 4.5 mm2. Finely ground samples were
sandwiched between Kapton tape and positioned at 451 to the
X-ray beam. Spectra were measured in transmission mode with
an ionization detector (filled with a 50:50 mixture of He and N2
in the ionization chamber) and in fluorescence mode with a
Canberra 13-element fluorescence detector. The step size was
0.1 eV through the absorption edge. Standards of Zr or Cu metal
were placed in a second position downstream of the samples and
measured in transmission mode concurrently with the samples.
The maxima in the first derivatives of the K-edges were set to
17,998 eV for Zr and 8979 eV for Cu [13]. All XANES spectra were
analyzed with use of the Athena software program [13].
2.4. Band structure calculations
Tight-binding linear muffin tin orbital (TB-LMTO) band
structure calculations were performed on ZrCuSiAs and ZrCuSiP
within the local density and atomic spheres approximations using
the Stuttgart TB-LMTO program [14]. The basis sets consisted of Zr
5s/5p/4d/4f, Cu 4s/4p/3d, Si 3s/3p/3d, As 4s/4p/4d, and P 3s/3p
orbitals, with the Zr 4f, Si 3d, and As 4d orbitals being downfolded.
Integrations in reciprocal space were carried out with an
improved tetrahedron method over 84 independent k points
Fig. 2. Zr 3d XPS spectra, fitted with 3d5/2 and 3d3/2 components, for ZrCuSiAs,
ZrCuSiP, and Zr metal. The vertical dashed line marks the 3d5/2 BE for Zr metal.

ARTICLE IN PRESS
P.E.R. Blanchard et al. / Journal of Solid State Chemistry 183 (2010) 1536–1544
1539
empty conduction states above the Fermi edge [16]. In this way,
3.2. [CuPn] layer
the core electrons experience substantial nuclear screening to
the extent that the Zr 3d BEs in ZrCuSiAs and ZrCuSiP do not
shift much from that in Zr metal. Further evidence for this
electronic delocalization is provided by resistivity measurements,
which show semimetallic behaviour for ZrCuSiP [8], and band
structure calculations, which show a pseudogap at the Fermi level
(vide infra).
The Si 2p XPS spectra were fitted to two components
representing the 2p3/2 and 2p1/2 spin–orbit-split final states, in
an intensity ratio of 2:1, FWHM of 0.9 eV, and an energy splitting
of 0.6 eV (Fig. 3a). (The second set of peaks at higher BE is assigned
to surface silicon suboxides [17,18].) Surprisingly, the Si 2p3/2 BEs
in ZrCuSiAs (99.41(4) eV) and ZrCuSiP (99.40(5) eV) are, within
standard uncertainty, the same as in elemental Si (99.5(2) eV)
[15,17], even though the formal charge is 2ꢀ. A closely related
case is Zr(As0.6Si0.4)As, in which the more perceptibly shifted Si
2p3/2 BE (98.9 eV) is correlated with an approximate negative
charge of 1ꢀ [18]. The implication is that the Zr–to–Si charge
transfer is less pronounced and that the Zr–Si bonds are even
more highly covalent in ZrCuSiAs and ZrCuSiP than in
The Cu 2p XPS spectra for ZrCuSiAs and ZrCuSiP consist of 2p3/2
and 2p1/2 core-line peaks in an intensity ratio of 2:1 (Fig. 4a).
Interestingly, despite the expectation of Cu1+, these spectra
exhibit features characteristic of both Cu2+ and Cu1+ systems.
On one hand, the Cu 2p3/2 BEs in ZrCuSiAs (933.68(3) eV) and
ZrCuSiP (933.70(6) eV) are higher than in Cu metal (932.86(4) eV).
The occurrence of BE shifts relative to Cu metal is usually
diagnostic for compounds containing Cu2+ but not Cu1+ [22–25].
On the other hand, as gauged by their FWHMs, the Cu 2p3/2 peaks
in ZrCuSiAs and ZrCuSiP (1.0 eV) are much narrower than in CuO
(3.2 eV) but closer to those in CuBr or CuI (1.2 eV) [26–28]. The
absence of line broadening is usually diagnostic for compounds
containing Cu1+ but not Cu2+. Significant line broadening occurs
for Cu2+ (3d9) where the interaction of unpaired 3d electrons with
2p electrons after photoionization gives rise to multiplet splitting,
but not for Cu1+ (3d10) where no such interaction can occur
because of the absence of unpaired electrons [29]. These seeming
contradictions can be reconciled upon realization that the
bonding in these quaternary pnictides is highly covalent. Thus,
the BE correlation may break down, similar to the case of the
chalcogenides Cu2S and CuS, which are indistinguishable by their
Cu 2p BEs [29]. Moreover, because these pnictides are expected to
be semimetallic, the mechanism for line broadening is different.
The Cu 2p3/2 peaks actually resemble more closely those in Cu
metal (FWHM of 0.8 eV), CeCu4Al (FWHM of 1.0 eV) [30], and
Zr(As0.6Si0.4)As. The absence of
a BE shift would also be
consistent with the occurrence of strong Si–Si bonding within
square nets in the structure, supporting the presence of an
extended polyanionic Si network. The identical BEs in ZrCuSiAs
and ZrCuSiP suggest that the Si charges are independent of the
nature of the pnicogen. To confirm this point, the Si L-edge XANES
spectra, which are less sensitive to final state effects than are XPS
spectra [19], were analyzed (Fig. 3b). The features (A–E) in these
spectra arise from dipole-allowed transitions of 2p electrons into
other alloys containing Cu1+ or Cu0 [25], with a small asymmetry
ˇ
´
to higher BE arising in the same way (Doniach–Sunjic process) as
described earlier for the Zr 3d peaks.
unoccupied s and d states (
tetrahedrally coordinated Si (such as in
D
l¼ 71) and are characteristic of
Another defining feature that can be examined is an intense
satellite above the 2p3/2 core line at 940–945 eV seen in the Cu 2p
XPS spectra for Cu2+ but not Cu1+ or Cu0 systems [22–24,29], as
has been exploited to identify Cu2+ in cuprate superconductors
[26,31–37]. The two theories developed (a ‘‘shake-up’’ process in
which Cu valence electrons are excited into empty ligand states
above the Fermi edge after photoionization [24,29], or a ‘‘shake-
down’’ process in which ligand electrons relax into empty
conduction states after photoionization [29]) both predict that
this satellite is only possible in Cu2+ systems where empty 3d
a
-quartz) [20]. Peaks A
and B correspond to transitions from 2p to 3s states (with a1
symmetry); they are split 0.6 eV apart from spin–orbit-coupling
into 2p3/2 (L3-edge) and 2p1/2 states [20]. As anticipated, the
L-edge absorption energies are essentially identical for both
ZrCuSiAs and ZrCuSiP. Peak C is assigned to a t2-tn2 (2p to strongly
hybridized s–p states) transition, peak D to multiple scattering
resonance (MSR), and peak E to a 2p-3d transition (or possibly a
surface oxide) [20,21].
Fig. 3. (a) Si 2p XPS spectra, fitted with 2p3/2 and 2p1/2 components, and (b) Si L-edge XANES spectra (FLY mode) for ZrCuSiAs and ZrCuSiP. The vertical dashed line in (a)
marks the 2p3/2 BE for elemental Si. Features A–E in (b) are discussed in Section 3.1.

ARTICLE IN PRESS
1540
P.E.R. Blanchard et al. / Journal of Solid State Chemistry 183 (2010) 1536–1544
Fig. 4. (a) Cu 2p XPS spectra and (b) Cu L-edge XANES spectra (FLY mode) for ZrCuSiAs, ZrCuSiP, and Cu metal. The vertical dashed lines mark the Cu 2p3/2 BE in (a) and the
Cu L3-edge absorption energy in (b) for Cu metal. Spectra are offset for clarity.
conduction states are available. The absence of this satellite in
ZrCuSiAs and ZrCuSiP thus eliminates the possibility for an
The magnitude of these shifts should reflect the degree of
electron transfer within the Cu–Pn bonds. Interestingly, the BE
assignment of Cu2+
.
shifts are less pronounced for the As atoms (DBE of 0.6 eV) than for
Further information can be gained from the Cu L-edge
XANES spectra, which reveal dipole-allowed transitions from
2p3/2 (L3-edge) or 2p1/2 (L2-edge) states to unoccupied 4s and 3d
states (Fig. 4b). The lineshapes are typical of Cu1+ compounds.
Given the d10 configuration for Cu1+ and Cu0, the 2p-4s
transition is much more probable than the 2p-3d transition,
whereas the reverse holds for Cu2+ (3d9) [38]. The correlation
between absorption energy and charge is not so straightforward.
Although it is true that the L3-edge energy for Cu1+ is higher
than for Cu0 because of the greater positive charge, it is actually
lower for Cu2+ because of the presence of unfilled 3d states [38].
The L3-edge absorption energy in ZrCuSiAs and ZrCuSiP (both
934.0 eV) is found to be higher than in Cu metal (932.7 eV),
supporting the presence of Cu1+. In principle, the intensity of the
Cu L-edge, which should be proportional to the number of
conduction states, can be analyzed; however, this relationship is
not valid here because these spectra were measured in FLY mode,
which is influenced by self-absorption effects.
The As 3d and P 2p XPS spectra each show one broad signal,
fitted to the As 3d5/2 and 3d3/2 components (intensity ratio of 3:2,
FWHM of 0.9 eV, splitting of 0.7 eV) and P 2p3/2 and 2p1/2
components (intensity ratio of 2:1, FWHM of 0.7 eV, splitting
of 0.9 eV), respectively (Fig. 5). The As XPS spectrum shows a small
asymmetry at higher BE which is attributed to surface oxides
(as seen in the Zr XPS spectrum, as well as in As1–xSex [39]) or may
be caused by the line-broadening mechanism described earlier
involving electronic delocalization in ZrCuSiAs. The P XPS spectrum
includes a minor component which is assigned to the 2p1/2 core-
line peak of trace amounts of elemental P, with the 2p3/2 peak
presumably buried under the more intense main peaks for ZrCuSiP.
The As 3d5/2 BE is lower in ZrCuSiAs (41.13(6) eV) than in elemental
As (41.7(2)eV) and the P 2p3/2 BE is lower in ZrCuSiP (128.82(6) eV)
than in elemental P (130.2(3) eV) [15,17], indicating the presence
of anionic pnicogen atoms in both cases. As in the case for the
Si atoms within the [ZrSi] layer, these BE shifts are less extreme
than expected for nominal Pn3ꢀ anions within the [CuPn] layer
because of the substantial covalent character in the bonds.
the P atoms ( BE of 1.4 eV), implying that As is less electronegative
D
than P. This observation contradicts the relative ordering of the
Allred–Rochow electronegativities for As (2.20) and P (2.06) [40],
as was previously noted in a comparative XPS study of binary
transition-metal phosphides vs. arsenides [41].
3.3. Charge transfer within and between [ZrSi] and [CuPn] layers
It is useful to summarize the conclusions reached so far. The
[ZrSi] layer contains Zr cations and Si anions, but the charges are
not extreme (as implied by little or even no energy shifts relative
to the element) and there is evidence for electronic delocalization.
The [CuPn] layer contains Cu1+ cations and Pn anions, with more
pronounced energy shifts, consistent with
a greater ionic
character in the Cu–Pn bonds than in the Zr–Si bonds. Of the
remaining questions, the most important is whether the separate
layers interact; that is, we want to address the possibility that the
Pn atoms are bonded not only to the Cu atoms, but also to the Zr
atoms (Fig. 1b). If so, then it is of interest to determine how the
extent of Zr–to–Pn or Cu–to–Pn charge transfer is altered in
ZrCuSiAs vs. ZrCuSiP. Because transitions in Zr and Cu XANES
spectra can reflect changes in the occupancy of specific states
within atoms, they provide a way to answer these questions.
The Zr K-edge spectra reveal transitions of 1s electrons into
unoccupied p states (Fig. 6a). The immediate observation that the
spectra differ for ZrCuSiAs and ZrCuSiP implies that the Zr and Pn
atoms do interact. We use the calculated conduction band for
ZrCuSiAs (Fig. 7) to aid in the interpretation of these spectra by
identifying qualitatively what orbital contributions are likely in
the final states of these transitions. (The calculated conduction
band for ZrCuSiP is similar and not shown here.) Peak A is
assigned as a transition from Zr 1s to Zr 5p states that are
hybridized with p states of Si and Pn atoms. This peak is more
intense in ZrCuSiAs than in ZrCuSiP, which indicates that there are
more pnicogen-based conduction states available in the former,
consistent with the earlier assertion that As is less electronegative

ARTICLE IN PRESS
P.E.R. Blanchard et al. / Journal of Solid State Chemistry 183 (2010) 1536–1544
1541
Fig. 5. (a) As 3d XPS spectrum, fitted with 3d5/2 and 3d3/2 components, for ZrCuSiAs, and (b) P 2p XPS spectrum, fitted with 2p3/2 and 2p1/2 components, for ZrCuSiP.
The vertical dashed lines mark the As 3d5/2 BE for elemental As and the P 2p3/2 BE for elemental P.
Fig. 6. Normalized (a) Zr K-edge, (b) Zr L-edge, and (c) Cu K-edge XANES spectra for ZrCuSiAs and ZrCuSiP. Zr and Cu K-edge spectra were measured in transmission mode
and Zr L-edge spectra in TEY mode. Marked features are discussed in Section 3.3.
than P and thus that charge transfer is less from Zr to As than to
P atoms. (An alternative assignment, an MSR phenomenon in
which the peak becomes more intense with larger next-nearest
neighbour atoms [42], can be dismissed because the second
coordination sphere around Zr is the same in both compounds.)
Peak B is assigned either as a transition from Zr 1s to Zr 5p states
that are hybridized with Cu 4p states, or possibly an MSR process
[42]. More convincing evidence for the interaction of Zr and Pn
atoms is provided by the Zr L-edge spectra, which probe the Zr 4d
states directly involved in bonding. Here, the spectra consist of

ARTICLE IN PRESS
1542
P.E.R. Blanchard et al. / Journal of Solid State Chemistry 183 (2010) 1536–1544
L3- and L2-edges, corresponding to transitions from Zr 2p3/2 and
2p1/2 states, respectively, to unoccupied 4d states (Fig. 6b). Both
edges are less prominent in ZrCuSiAs than in ZrCuSiP, indicating
that there are fewer Zr-based conduction states in the former.
Complementary information about the extent of metal-to-
pnicogen charge transfer can be gained from the Cu K-edge
XANES spectra, which reveal transitions of Cu 1s electrons into
unoccupied p states (Fig. 6c). Peak A resembles a pre-edge feature
corresponding to a dipole-forbidden 1s-3d transition commonly
found for Cu2+ species by virtue of their unfilled 3d-based
conduction states [43]. Because the possibility of Cu2+ was
already eliminated from the XPS data, the more likely assignment
is a dipole-allowed transition from Cu 1s to strongly hybridized
Cu 3d/4p states associated with the tetrahedral coordination
geometry around Cu1+ species (similar to the case of copper(I)
halides) [44]. An alternative assignment is a transition from Cu 1s
to Cu 4p states that are hybridized with Zr 4d states, as found near
the Fermi edge in the conduction band (Fig. 7). Although the Zr
˚
and Cu atoms are more than 3 A apart, similar types of long-
distance interactions have been implicated in the spectra of
Ca2–xSrxFe2O5 [45], LaFeAsO1–xFx [46], and La2CuO4 [47]. Peak B is
assigned as a transition from Cu 1s to Si 3p and Pn np states.
With the assumption that the contribution from the Si states is
identical (on the basis of the lack of changes in the Si XPS and
XANES spectra), the greater intensity of this peak in ZrCuSiAs than
in ZrCuSiP confirms that there are more pnicogen-based conduc-
tion states in the former. As before, this implies that As is less
electronegative than P and that charge transfer is less from Cu to
As than to P atoms. The remaining peaks, C and D, are assigned as
transitions from Cu 1s to Zr 5p and Cu 4p states (seen in the
conduction band), or possibly as MSR phenomena [48].
3.4. Band structure
It is now of interest to see if the conclusions gained from
the spectroscopic evidence are supported by the calculated
band structure for ZrCuSiAs, in comparison to a representative
rare-earth arsenide oxide, LaNiAsO. In common with other
Fig. 7. Orbital projections of the calculated conduction states for ZrCuSiAs. The
projections are offset for clarity but are set on the same scale.
Fig. 8. Density of states (DOS) with its orbital projections and crystal orbital Hamilton population (COHP) curves for (a, b) LaNiAsO and (c, d) ZrCuSiAs. The Fermi level
is at 0 eV.

ARTICLE IN PRESS
P.E.R. Blanchard et al. / Journal of Solid State Chemistry 183 (2010) 1536–1544
1543
rare-earth pnictide oxides, the band structure for LaNiAsO is
characterized by a partially filled valence band that comprises a
narrow range of La- and O-based states (from ꢀ6 to ꢀ3 eV) and a
wider range of Ni- and As-based states (from ꢀ6 eV upwards),
with the bands near the Fermi level dominated by Ni 3d states
(Fig. 8a). (The narrow low-energy band centred near ꢀ11.5 eV
largely comprises As 4s states.) Notwithstanding the usual
assumption that the [LaO] layer contains ionic bonds [5], the
mixing of La and O states implies that there is also some covalent
character here, as reflected by the modest integrated crystal
orbital Hamilton population (–ICOHP) value of 0.65 eV/bond and
the occupation of La–O bonding levels in this energy range
(Fig. 8b). The main stabilizing contribution in this structure,
however, occurs within the [NiAs] layer, where a large –ICOHP
of 1.88 eV/bond for the Ni–As contacts is derived from the
occupation of strongly bonding levels. Weak Ni–Ni bonding
(–ICOHP of 0.17 eV/bond) is associated with the arrangement
and highly covalent M–As bonds. The [LaO] and [NiAs] layers in
LaNiAsO can be treated independently (as suggested by the
insensitivity of the As 3d BEs upon substitution of La with other RE
elements), but the [ZrSi] and [CuAs] layers in ZrCuSiAs are
strongly interacting (as suggested by the visible changes in the Zr
K- and L-edge XANES spectra upon substitution of As with P). That
is, there exists a real charge transfer from Zr atoms in the [ZrSi]
layer to the As atoms in the [CuAs] layer. As inferred from shifts in
the Zr K- and L-edge absorption energies, this metal-to-pnicogen
charge transfer is enhanced even more in ZrCuSiP, indicating that
P is more electronegative than As (in contradiction to tabulated
Allred–Rochow values). Band structure calculations confirm that
Si–Si and Zr–As interactions are ultimately responsible for the
development of a three-dimensional covalent bonding network
in ZrCuSiAs, in contrast to the two-dimensional character in
LaNiAsO. The isolation of insulating (ionic) and conducting
(covalent) layers is believed to be important in imparting the
superconducting behaviour of the layered pnictide oxides.
Strengthening the interactions between these layers can lead to
changes in electronic structure that are manifested, for example,
˚
of Ni atoms 2.92 A apart forming square nets within the [NiAs]
layer. The corresponding arrangement of O atoms, too far apart
˚
at 2.92 A, within the [LaO] layer leads to no O–O bonding
interactions (–ICOHP of 0.02 eV/bond), of course. If the
interaction between La and As atoms is interrogated, the small
(but non-negligible) –ICOHP value of 0.10 eV/bond indicates that
interlayer bonding between the [LaO] and [NiAs] layers is
considerably weaker than intralayer bonding; the picture of
nearly separate ionic [LaO] and covalent [NiAs] layers holds.
The band structure for ZrCuSiAs reveals some striking
differences (Fig. 8c). The Fermi level falls in a pseudogap, so that
semimetallic behaviour is predicted. (Although the properties
of ZrCuSiAs have not been measured, the electrical resistivity of
by smaller optical band gaps in b-PrZnPO than in a-PrZnPO
[49,50]. It may be interesting to think of ways of tuning the
transition from two- to three-dimensional character, something
that appears to be feasible with this structure type.
Acknowledgments
This work was supported through Discovery Grants to A.M.
and R.G.C. from the Natural Sciences and Engineering Research
Council (NSERC) of Canada. P.E.R.B thanks NSERC, Alberta
Ingenuity, and the University of Alberta for scholarship support.
Access to the Kratos AXIS 165 XPS spectrometer was provided by
the Alberta Centre for Surface Engineering and Science (ACSES),
which was established with support from the Canada Foundation
for Innovation (CFI) and Alberta Innovation and Science. We thank
Mr. Thomas Regier (SGM beamline), Dr. Lucia Zuin (PGM beam-
line), and Dr. Yongfeng Hu (PGM and SXRMB beamlines) for
assistance with the XANES experiments conducted at the CLS,
which is supported by NSERC, NRC, CIHR, and the University of
Saskatchewan. We thank Dr. Robert Gordon for assistance with
the Zr and Cu XANES experiments at PNC/XOR-CAT facilities at the
APS. The PNC/XOR facilities are supported by the US Department
of Energy—Basic Energy Sciences, a major research support grant
(MRS) from NSERC, the University of Washington, Simon Fraser
University, and the APS. The APS is also supported by the US
Department of Energy, Office of Science, Office of Basic Energy
Sciences, under Contract DE-AC02-06CH11357.
ZrCuSiP is quite high, 1.5 ꢁ 10–3
O cm at 300 K, and falls only to
Ocm at 5 K [8].) Unlike the [LaO] layer in LaNiAsO, the
1.0 ꢁ 10–3
[ZrSi] layer gives rise to dramatically wider energy dispersion,
consistent with more pronounced covalent bonding character
associated with the occupation of Zr–Si bonding levels (–ICOHP of
1.51 eV/bond) (Fig. 8d). A wide subband extending down to
ꢀ11 eV originates from orbital interactions of Si-based states,
˚
made possible by the closer 2.60 A separation of Si atoms within
square nets in the [ZrSi] layer. Occupation of Si–Si bonding levels
leads to a substantial –ICOHP of 1.41 eV/bond, confirming the
presence of strong delocalized homoatomic bonding within these
polyanionic Si square nets. Relative to the [NiAs] layer in LaNiAsO,
the compression of the [CuAs] layer in ZrCuSiAs within the ab
plane does not alter the metal-arsenic bonding too much (–ICOHP
of 1.43 eV per Cu–As bond) but does enhance the metal–metal
bonding (–ICOHP of 0.49 eV per Cu–Cu bond). Importantly, the
occupation of Zr–As bonding levels leads to substantial interlayer
bonding (–ICOHP of 1.73 eV/bond) that is of the same magnitude
as the intralayer bonding. Consistent with the Zr XANES spectra
which show evidence for Zr–As interactions, the picture of
interacting [ZrSi] and [CuAs] layers is confirmed.
Appendix A. Supplementary materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.jssc.2010.04.032.
4. Conclusions
A comparison of XPS and XANES spectra reveals profound
differences that reflect the enhancement of covalent character on
progressing from LaNiAsO to ZrCuSiAs. The [LaO] layer in LaNiAsO
exhibits significant ionic character, as indicated by its O 1s BE
which falls in the range expected for normal oxides, whereas the
[ZrSi] layer in ZrCuSiAs exhibits strongly covalent character, as
indicated by its Si 2p BE which is nearly the same as in elemental
Si and is consistent with the development of a polyanionic Si
network. The [NiAs] layer in LaNiAsO and the [CuAs] layer in
ZrCuSiAs are similar in nature, with As 3d5/2 BEs slightly lower
than in elemental As, indicating the presence of anionic As atoms
References
[1] R. Po¨ttgen, D. Johrendt, Z. Naturforsch. B 63 (2008) 1135–1148.
[2] B.I. Zimmer, W. Jeitschko, J.H. Albering, R. Glaum, M. Reehuis, J. Alloys Compd.
229 (1995) 238–242.
[3] Y.-W. Ma, Z.-S. Gao, L. Wang, Y.-P. Qi, D.-L. Wang, X.-P. Zhang, Chin. Phys. Lett.
26 (2009) 037401-1–037401-4.
[4] Z.A. Ren, J. Yang, W. Lu, W. Yi, G.C. Che, X.L. Dong, L.L. Sun, Z.X. Zhao, Mater.
Res. Innov. 12 (2008) 105–106.
[5] H. Takahashi, K. Igawa, K. Arii, Y. Kamihara, M. Hirano, H. Hosono, Nature 453
(2008) 376–378.

ARTICLE IN PRESS
1544
P.E.R. Blanchard et al. / Journal of Solid State Chemistry 183 (2010) 1536–1544
[6] H. Hiramatsu, H. Yanagi, T. Kamiya, K. Ueda, M. Hirano, H. Hosono, Chem.
Mater. 20 (2008) 326–334.
[7] V. Johnson, W. Jeitschko, J. Solid State Chem. 11 (1974) 161–166.
[8] H. Abe, K. Yoshii, J. Solid State Chem. 165 (2002) 372–374.
[9] P.E.R. Blanchard, B.R. Slater, R.G. Cavell, A. Mar, A.P. Grosvenor, Solid State Sci.
12 (2009) 50–58.
[28] R.P. Vasquez, Surf. Sci. Spectra 2 (1993) 149–154.
[29] S.K. Chawla, N. Sankarraman, J.H. Payer, J. Electron Spectrosc. Relat. Phenom.
61 (1992) 1–18.
[30] A. Kowalczyk, T. Tolin´ski, M. Reiffers, M. Pugaczowa-Michalska, G. Che"kowska
ˇ
E. Gazo, J. Phys.: Condens. Matter 20 (2008) 255252-1–255252-7.
[31] P.C. Healy, S. Myhra, A.M. Stewart, Jpn. J. Appl. Phys. 26 (1987) L1884–L1887.
[32] H. Ishii, T. Koshizawa, T. Hanyu, S. Yamaguchi, Jpn. J. Appl. Phys. 32 (1993)
1070–1076.
[10] D. Briggs, Surface Analysis of Polymers by XPS and Static SIMS, Cambridge
University Press, Cambridge, 2005.
[11] N. Fairley, CasaXPS, Version 2.3.9, Casa Software Ltd., Teighnmouth, Devon,
[33] J. Sugiyama, R. Itti, H. Yamauchi, N. Koshizuka, S. Tanaka, Phys. Rev. B 45
(1992) 4952–4956.
UK, 2003 /www.casaxps.comS.
[12] G. Canning, M. Kasrai, G.M. Bancroft, D. Andrew, J. Synchrotron Radiat. 6
(1999) 740–742.
[34] R.P. Vasquez, M.P. Siegal, D.L. Overmyer, Z.F. Ren, J.Y. Lao, J.H. Wang, Phys.
Rev. B 60 (1999) 4309–4319.
[13] B. Ravel, M. Newville, J. Synchrotron Radiat. 12 (2005) 537–541.
[14] R. Tank, O. Jepsen, A. Burkhardt, O.K. Andersen, TB-LMTO-ASA Program, Version
4.7, Max Planck Institut fu¨r Festko¨rperforschung, Stuttgart, Germany, 1998.
[15] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell,
J.R. Rumble Jr., NIST X-ray Photoelectron Spectroscopy Database, Version
3.5 (web version), National Institute of Standards and Technology, Gaithers-
burg, MD, 2003 /srdata.nist.gov/xpsS.
[35] P. Steiner, V. Kinsinger, I. Sander, B. Siegwart, S. Hu¨fner, C. Politis, Z. Phys. B:
Condens. Matter 67 (1987) 19–23.
[36] A. Bianconi, A. Congiu Castellano, M. De Santis, P. Rudolf, P. Lagarde
A.M. Flank, A. Marcelli, Solid State Commun. 63 (1987) 1009–1013.
[37] J.-H. Choy, S.-H. Choi, S.-H. Byeon, S.-H. Chun, S.-T. Hong, D.-Y. Jung
W.-Y. Choe, Y.-W. Park, Bull. Korean Chem. Soc. 9 (1988) 289–291.
[38] R.A.D. Pattrick, G. van der Laan, J.M. Charnock, B.A. Grguric, Am. Mineral. 89
(2004) 541–546.
ˇ
´
[16] S. Doniach, M. Sunjic, J. Phys. C: Solid State Phys. 3 (1970) 285–291.
[17] A.P. Grosvenor, R.G. Cavell, A. Mar, R.I.R. Blyth, J. Solid State Chem. 180 (2007)
2670–2681.
[39] A. Siokou, M. Kalyva, S.N. Yannopoulos, M. Frumar, P. Ne˘mee, J. Phys.:
Condens. Matter 18 (2006) 5525–5534.
[18] M.W. Gaultois, A.P. Grosvenor, P.E.R. Blanchard, A. Mar, J. Alloys Compd. 492
(2009) 19–25.
[19] R.L. Opila, G.D. Wilk, M.A. Alam, R.B. van Dover, B.W. Busch, Appl. Phys. Lett.
81 (2002) 1788–1790.
[20] D. Li, G.M. Bancroft, M. Kasrai, M.E. Fleet, X.H. Feng, K.H. Tan, B.X. Yang, Solid
State Commun. 87 (1993) 613–617.
[40] A.L. Allred, E.G. Rochow, J. Inorg. Nucl. Chem. 5 (1958) 264–268.
[41] A.P. Grosvenor, R.G. Cavell, A. Mar, J. Solid State Chem. 181 (2008)
2549–2558.
[42] F. Farges, S. Rossano, Eur. J. Mineral. 12 (2000) 1093–1107.
[43] J.M. Brown, L. Powers, B. Kincaid, J.A. Larrabee, T.G. Spiro, J. Am. Chem. Soc.
102 (1980) 4210–4216.
[21] M. Kasrai, W.N. Lennard, R.W. Brunner, G.M. Bancroft, J.A. Bardwell, K.H. Tan,
Appl. Surf. Sci. 99 (1996) 303–312.
[44] T. Chattopadhyay, A.R. Chetal, J. Phys. C: Solid State Phys. 18 (1985)
5373–5378.
[22] D.C. Frost, A. Ishitani, C.A. McDowell, Mol. Phys. 24 (1972) 861–877.
[23] T. Ghodselahi, M.A. Vesaghi, A. Shafiekhani, A. Baghizadeh, M. Lameii, Appl.
Surf. Sci. 255 (2008) 2730–2734.
[24] A. Rosencwaig, G.K. Wertheim, J. Electron Spectrosc. Relat. Phenom. 1 (1972/
73) 493–496.
[25] A. Roberts, D. Engelberg, Y. Liu, G.E. Thompson, M.R. Alexander, Surf. Interface
Anal. 33 (2002) 697–703.
[26] F. Werfel, M. Heinonen, E. Suoninen, Z. Phys. B: Condens. Matter 70 (1988)
317–322.
[45] A.P. Grosvenor, J.E. Greedan, J. Phys. Chem. C 113 (2009) 11366–11372.
[46] A. Koitzsch, D. Inosov, J. Fink, M. Knupfer, H. Eschrig, S.V. Borisenko, G. Behr,
A. Ko¨hler, J. Werner, B. Bu¨chner, R. Follath, H.A. Du¨rr, Phys. Rev. B 78 (2008)
180506-1–180506-4.
[47] S.K. Pandey, S. Khalid, A.V. Pimpale, J. Phys.: Condens. Matter 19 (2007)
036212-1–036212-16.
[48] J.E. Penner-Hahn, Coord. Chem. Rev. 190–192 (1999) 1101–1123.
[49] H. Lincke, T. Nilges, R. Po¨ttgen, Z. Anorg. Allg. Chem. 632 (2006) 1804–1808.
[50] H. Lincke, R. Glaum, V. Dittrich, M. Tegel, D. Johrendt, W. Hermes, M.H. Mo¨ller
T. Nilges, R. Po¨ttgen, Z. Anorg. Allg. Chem. 634 (2008) 1339–1348.
[27] R.P. Vasquez, Surf. Sci. Spectra 2 (1993) 144–148.