A citation index is a bibliographic index recording citations between publications, allowing the user to establish which later documents cite (i.e. contain references to) earlier documents. Several citation indexes are already available, some of which are freely accessible but not downloadable (e.g. Google Scholar), while others can be accessed only by paying significant subscription/access fees (e.g. Web of Science and Scopus).
OpenCitations, as an infrastructure organization for open scholarship, has built the OpenCitations Index, a totally open citation index built on the bibliographic metadata for citing and cited publications that are stored in a separate database, OpenCitations Meta, using information available from a number of open bibliographic databases.
The current sources used for building the OpenCitations Index are:
Citations are treated as first-class data entities, with accompanying properties – for a full explanation, see our introductory blog post and following posts. In particular, each citation is identified by an Open Citation Identifier (OCI), which has a simple structure: the lower-case letters "oci" followed by a colon, followed by two numbers separated by a dash (e.g. oci:060401-06103230153). OCIs can be resolved using the OpenCitations OCI Resolution Service.
The citation metadata within the OpenCitations Index are recorded in RDF. The RDF statements are organised according to the OpenCitations Data Model.
All the data in the OpenCitations Index are available for download in the following ways:
by querying the OpenCitations Index SPARQL endpoint;
by using the OpenCitations Index REST API, implemented by means of RAMOSE (the Restful API Manager Over SPARQL Endpoints), where they can be downloaded in JSON and CSV formats;
as dumps of the full index on Figshare in CSV, N-Triples and Scholix formats;
using the HTTP URI of the individual citations, where they can be downloaded in different formats (HTML, RDF/XML, Turtle, and JSON-LD) via content negotiation.
Additionally, the content of the OpenCitations Index can be searched using OSCAR and LUCINDA, the search and browse interfaces developed by OpenCitations.