Changing the physical and chemical properties of titanium oxynitrides TiN
1-x
O
x
by changing the composition
Jesús Graciani,
1
Said Hamad,
2,3
and Javier Fdez. Sanz
1,
*
1
Departamento de Química Física, Facultad de Química, Universidad de Sevilla, E-41012 Sevilla, Spain
2
Instituto de Ciencia de Materiales de Sevilla, CSIC–Universidad de Sevilla, Avda. Américo Vespucio 49, 41092 Sevilla, Spain
3
Department of Physical, Chemical and Natural Systems, University Pablo de Olavide, Carretera de Utrera, km 1, 41013 Sevilla, Spain
Received 7 June 2009; published 18 November 2009
The stability and structural properties of titanium oxynitrides, TiN
1-x
O
x
, of different compositions are
theoretically analyzed by means of first-principles periodic density-functional calculations. We show that at
x =0.55–0.6 there is a change in the preferred structure from that of NaCl type to the -TiO arrangement. For
the NaCl-type structure the cell volume increases with x while it decreases with x for the -TiO structure. The
bulk moduli are always much larger for NaCl-type structures than for -TiO and they decrease as the amount
of O increases, moving from 280 GPa for TiN to 226 GPa for TiO NaCl-type structure or 197 GPa for -TiO.
Changes in the electronic structure with the composition are also considered. In general we found that in the
two types of structure NaCl and -TiO, both the band gap and the ionic character increase with the O
concentration.
DOI: 10.1103/PhysRevB.80.184112 PACS numbers: 61.50.Ah, 71.15.Mb, 81.05.Je
I. INTRODUCTION
Titanium nitride is widely known as a refractory hard
metal since it exhibits an extraordinary combination of
properties:
1,2
i ultrahardness nearing that of diamond and
high melting point; ii brittleness, high thermal and electri-
cal conductivities higher than that of titanium metal, and
even low-temperature superconductivity,
3,4
and iii a NaCl-
type ionic structure. Such an unusual combination of cova-
lent, metallic, and ionic properties makes it a good candidate
for technological applications in many areas as
microelectronics,
5–15
wear resistant coatings on cutting
tools,
16,17
dental surgery,
18
decorative applications,
19
and as
potential sensors and catalysts.
20,21
On the other hand, titanium dioxide is a well-known semi-
conductor with many applications in some of the most im-
portant current research areas, such as solar energy
harvesting,
22
photocatalysis,
23–27
and heterogeneous catalysis
of supported metal nanoclusters.
28–32
The easiness of prepa-
ration and stability of TiO
2
surfaces, in particular, the 110
face of rutile, make this material a real paradigm in surface
science and catalysis.
A number of intermediate phases of general composition
TiO
x
N
y
called “oxynitrides” are found midway between TiN
and TiO
2
. Obviously the properties of the oxynitrides will be
similar to those of the respective pure nitride and oxide when
their compositions are close to those of the pure systems and
are expected to change progressively from those of the ni-
tride to those of the oxide and vice versa when the compo-
sitions move to intermediate values. However, concerning
these materials a number of points are open: are those oxyni-
trides stable phases, i.e., able to synthesize? What are their
structures? Are their properties a result of the combination of
those of the pure solids? Are we really able to control these
properties as a function of the composition TiO
x
N
y
?
Preparation of oxynitrides through either oxidation of TiN
or nitridation of TiO
2
constitutes a conspicuous problem. On
one hand, the oxidation of the TiN quickly leads to formation
of TiO
2
.
33,34
Only in some cases a very thin intermediate
phase of mixed composition and unidentified structure has
been observed between TiN and TiO
2
pure phases.
35,36
The-
oretical calculations have clearly shown that the oxidation of
TiN under ordinary oxygen pressures leads to surface recon-
structions of mixed composition and to the formation of
amorphous TiO
2
on the reconstructed layer.
18,34,37
Diffusion
of O atoms to the bulk TiN has never been observed to occur
in molecular-dynamics simulations, in fact the Ti atoms of
the TiN surface actually move to the reconstructed layer to
receive the incoming new oxygen molecules and form an
incipient TiO
2
.
37
Similarly, N implantation on TiO
2
leads to
large reconstructions of the surface due to a strong reduction
and the reachable amount of implanted N is only 2 – 3 %.
38
Previous density-functional theory DFT calculations
39
show that implanted N atoms need to be stabilized by an
electronic transfer going from formally N
2-
to the most
stable configuration N
3-
. Since the conduction band of TiO
2
is empty, the system needs to generate or to adsorb species
with the ability to transfer electrons to the implanted N at-
oms for example, oxygen vacancies, formally Ti
3+
ions, or
adsorbed metals.
39–43
No matter which preparation process we follow, oxidation
of TiN or nitridation of TiO
2
, the resulting structure is meta-
stable and not adequate to hold the optimal interactions be-
tween Ti and O atoms or N atoms. However, while there is
not any titanium nitride phase isostructural with TiO
2
, there
is a stable Ti-O phase isostructural with TiN.
44–48
This phase
of TiO, namely, -TiO, has a rock-salt structure in which an
ordered substructure of both oxygen and titanium vacancies
is superimposed on the original lattice. As a result the sym-
metry is shifted from cubic to monoclinic but the base rock-
salt-type pattern remains. Thus, it should be much easier to
synthesize an oxynitride of titanium from the nitridation of
TiO since the incorporated N atoms are already in their most
stable structure, namely, rock-salt structure. Moreover, in this
way, the main drawback for the implantation of N in TiO
2
,
the extra-electron transfer needed for closing the electronic
PHYSICAL REVIEW B 80, 184112 2009
1098-0121/2009/8018/18411210 ©2009 The American Physical Society 184112-1